[InstCombine] sink sext after ashr
Narrow ops are better for bit-tracking, and in the case of vectors,
may enable better codegen.
As the trunc test shows, this can allow follow-on simplifications.
There's a block of code in visitTrunc that deals with shifted ops
with FIXME comments. It may be possible to remove some of that now,
but I want to make sure there are no problems with this step first.
%s = sext i8 %x to i32 %r = ashr i32 %s, 3 ; shift value is < than source bit width => %a = ashr i8 %x, 3 %r = sext i8 %a to i32
%s = sext i8 %x to i32 %r = ashr i32 %s, 8 ; shift value is >= than source bit width => %a = ashr i8 %x, 7 ; so clamp this shift value %r = sext i8 %a to i32
%a = sext i16 %v to i32 %s = ashr i32 %a, 18 %t = trunc i32 %s to i16 => %t = ashr i16 %v, 15