LegalizeTypes support for EXTRACT_VECTOR_ELT. The

LegalizeTypes support for EXTRACT_VECTOR_ELT.  The
approach taken is different to that in LegalizeDAG
when it is a question of expanding or promoting the
result type: for example, if extracting an i64 from
a <2 x i64>, when i64 needs expanding, it bitcasts
the vector to <4 x i32>, extracts the appropriate
two i32's, and uses those for the Lo and Hi parts.
Likewise, when extracting an i16 from a <4 x i16>,
and i16 needs promoting, it bitcasts the vector to
<2 x i32>, extracts the appropriate i32, twiddles
the bits if necessary, and uses that as the promoted
value.  This puts more pressure on bitcast legalization,
and I've added the appropriate cases.  They needed to
be added anyway since users can generate such bitcasts
too if they want to.  Also, when considering various
cases (Legal, Promote, Expand, Scalarize, Split) it is
a pain that expand can correspond to Expand, Scalarize
or Split, so I've changed the LegalizeTypes enum so it
lists those different cases - now Expand only means
splitting a scalar in two.
The code produced is the same as by LegalizeDAG for
all relevant testcases, except for
2007-10-31-extractelement-i64.ll, where the code seems
to have improved (see below; can an expert please tell
me if it is better or not).
Before < vs after >.

<       subl    $92, %esp
<       movaps  %xmm0, 64(%esp)
<       movaps  %xmm0, (%esp)
<       movl    4(%esp), %eax
<       movl    %eax, 28(%esp)
<       movl    (%esp), %eax
<       movl    %eax, 24(%esp)
<       movq    24(%esp), %mm0
<       movq    %mm0, 56(%esp)
---
>       subl    $44, %esp
>       movaps  %xmm0, 16(%esp)
>       pshufd  $1, %xmm0, %xmm1
>       movd    %xmm1, 4(%esp)
>       movd    %xmm0, (%esp)
>       movq    (%esp), %mm0
>       movq    %mm0, 8(%esp)

<       subl    $92, %esp
<       movaps  %xmm0, 64(%esp)
<       movaps  %xmm0, (%esp)
<       movl    12(%esp), %eax
<       movl    %eax, 28(%esp)
<       movl    8(%esp), %eax
<       movl    %eax, 24(%esp)
<       movq    24(%esp), %mm0
<       movq    %mm0, 56(%esp)
---
>       subl    $44, %esp
>       movaps  %xmm0, 16(%esp)
>       pshufd  $3, %xmm0, %xmm1
>       movd    %xmm1, 4(%esp)
>       movhlps %xmm0, %xmm0
>       movd    %xmm0, (%esp)
>       movq    (%esp), %mm0
>       movq    %mm0, 8(%esp)

<       subl    $92, %esp
<       movaps  %xmm0, 64(%esp)
---
>       subl    $44, %esp

<       movl    16(%esp), %eax
<       movl    %eax, 48(%esp)
<       movl    20(%esp), %eax
<       movl    %eax, 52(%esp)
<       movaps  %xmm0, (%esp)
<       movl    4(%esp), %eax
<       movl    %eax, 60(%esp)
<       movl    (%esp), %eax
<       movl    %eax, 56(%esp)
---
>       pshufd  $1, %xmm0, %xmm1
>       movd    %xmm1, 4(%esp)
>       movd    %xmm0, (%esp)
>       movd    %xmm1, 12(%esp)
>       movd    %xmm0, 8(%esp)

<       subl    $92, %esp
<       movaps  %xmm0, 64(%esp)
---
>       subl    $44, %esp

<       movl    24(%esp), %eax
<       movl    %eax, 48(%esp)
<       movl    28(%esp), %eax
<       movl    %eax, 52(%esp)
<       movaps  %xmm0, (%esp)
<       movl    12(%esp), %eax
<       movl    %eax, 60(%esp)
<       movl    8(%esp), %eax
<       movl    %eax, 56(%esp)
---
>       pshufd  $3, %xmm0, %xmm1
>       movd    %xmm1, 4(%esp)
>       movhlps %xmm0, %xmm0
>       movd    %xmm0, (%esp)
>       movd    %xmm1, 12(%esp)
>       movd    %xmm0, 8(%esp)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47672 91177308-0d34-0410-b5e6-96231b3b80d8