[InstCombine] Fold select (icmp sgt x, -1), lshr (X, Y), ashr (X, Y) to ashr (X, Y))

Summary:
(select (icmp sgt x, -1), lshr (X, Y), ashr (X, Y)) -> ashr (X, Y))
(select (icmp slt x, 1), ashr (X, Y), lshr (X, Y)) -> ashr (X, Y))

Fixes PR41173

Alive proof by @lebedev.ri (thanks)
Name: PR41173
  %cmp = icmp slt i32 %x, 1
  %shr = lshr i32 %x, %y
  %shr1 = ashr i32 %x, %y
  %retval.0 = select i1 %cmp, i32 %shr1, i32 %shr
  =>
  %retval.0 = ashr i32 %x, %y

Optimization: PR41173
Done: 1
Optimization is correct!

Reviewers: lebedev.ri, spatel

Reviewed By: lebedev.ri

Subscribers: nikic, craig.topper, llvm-commits, lebedev.ri

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64285

llvm-svn: 365893