[InstCombine] try harder to form select from logic ops (2nd try)

The original patch was committed here:
rL344609
...and reverted:
rL344612
...because it did not properly check/test data types before calling
ComputeNumSignBits(). 

The tests that caused bot failures for the previous commit are 
over-reaching front-end tests that run the entire -O optimizer 
pipeline: 
    Clang :: CodeGen/builtins-systemz-zvector.c
    Clang :: CodeGen/builtins-systemz-zvector2.c

I've added a negative test here to ensure coverage for that case.
The new early exit check also tests the type of the 'B' parameter,
so we don't waste time on matching if either value is unsuitable.

Original commit message:

This is part of solving PR37549:
https://bugs.llvm.org/show_bug.cgi?id=37549

The patterns shown here are a special case of something
that we already convert to select. Using ComputeNumSignBits()
catches that case (but not the more complicated motivating
patterns yet).

The backend has hooks/logic to convert back to logic ops
if that's better for the target.

llvm-svn: 345149

0 files changed, 0 insertions, 0 deletions