[PowerPC] Revert r310346 (and followups r310356 & r310424) which

introduce a miscompile bug.

There appears to be a bug where the generated code to extract the sign
bit doesn't work correctly for 32-bit inputs. I've replied to the
original commit pointing out the problem. I think I see by inspection
(and reading the manual for PPC) how to fix this, but I can't be 100%
confident and I also don't know what the best way to test this is.
Currently it seems nearly impossible to get the backend to hit this code
path, but the patch autohr is likely in a better position to craft such
test cases than I am, and based on where the bug is it should be easily
done.

Original commit message for r310346:
"""
[PowerPC] Eliminate compares - add i32 sext/zext handling for SETLE/SETGE

Adds handling for SETLE/SETGE comparisons on i32 values. Furthermore, it
adds the handling for the special case where RHS == 0.

Differential Revision: https://reviews.llvm.org/D34048
"""

llvm-svn: 310809