[InstCombine] Liberate assert in InstCombiner::visitZExt

Summary:
The call to canEvaluateZExtd in InstCombiner::visitZExt may
return with BitsToClear == SrcTy->getScalarSizeInBits(), but
there is an assert that BitsToClear should be smaller than
SrcTy->getScalarSizeInBits().

I have a test case that triggers the assert, but it only happens
for my downstream target. I've not been able to trigger it for
any upstream target.

The assert triggered for a piece of code such as this
  %shr1 = lshr i16 undef, 15
  ...
  %shr2 = lshr i16 %shr1, 1
  %conv = zext i16 %shr2 to i32

Normally the lshr instructions are constant folded before we
visit the zext (that is why it is so hard to reproduce).
The original pattern, before instcombine, is of course a lot more
complicated in my test case. The shift count in the second lshr
is for example determined by the outcome of a PHI instruction.
It seems like other rewrites by instcombine leads up to
the pattern above. And then the zext is pulled from the
worklist, and visited (hitting the assert), before we detect
that the lshr instrucions can be constant folded.

Anyway, since the canEvaluateZExtd may return with BitsToClear
equal to SrcTy->getScalarSizeInBits(), and since the rewrite
that converts the expression type to avoid a zero extend works
also for the case where SrcBitsKept ends up being zero, then
it should be OK to liberate the assert to
  assert(BitsToClear <= SrcTy->getScalarSizeInBits() &&
         "Unreasonable BitsToClear");

Reviewers: hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D30993

llvm-svn: 297952