Better implementation of truncate. ISel matches it to a pseudo instruction

Better implementation of truncate. ISel matches it to a pseudo instruction
that gets emitted as movl (for r32 to i16, i8) or a movw (for r16 to i8). And
if the destination gets allocated a subregister of the source operand, then
the instruction will not be emitted at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28119 91177308-0d34-0410-b5e6-96231b3b80d8