Allow the live interval analysis pass to be a bit more aggressive about

Allow the live interval analysis pass to be a bit more aggressive about
numbering values in live ranges for physical registers.

The alpha backend currently generates code that looks like this:

  vreg = preg
...
  preg = vreg
  use preg
...
  preg = vreg
  use preg

etc.  Because vreg contains the value of preg coming in, each of the
copies back into preg contain that initial value as well.

In the case of the Alpha, this allows this testcase:

void "foo"(int %blah) {
        store int 5, int *%MyVar
        store int 12, int* %MyVar2
        ret void
}

to compile to:

foo:
        ldgp $29, 0($27)
        ldiq $0,5
        stl $0,MyVar
        ldiq $0,12
        stl $0,MyVar2
        ret $31,($26),1

instead of:

foo:
        ldgp $29, 0($27)
        bis $29,$29,$0
        ldiq $1,5
        bis $0,$0,$29
        stl $1,MyVar
        ldiq $1,12
        bis $0,$0,$29
        stl $1,MyVar2
        ret $31,($26),1

This does not seem to have any noticable effect on X86 code.

This fixes PR535.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20536 91177308-0d34-0410-b5e6-96231b3b80d8