Revert r229675 - [mips] Avoid redundant sign extension of the result of binary bitwis...

Revert r229675 - [mips] Avoid redundant sign extension of the result of binary bitwise instructions.

It introduced two regressions on 64-bit big-endian targets running under N32
(MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4, and
MultiSource/Applications/kimwitu++/kc) The issue is that on 64-bit targets
comparisons such as BEQ compare the whole GPR64 but incorrectly tell the
instruction selector that they operate on GPR32's. This leads to the
elimination of i32->i64 extensions that are actually required by
comparisons to work correctly.

There's currently a patch under review that fixes this problem.

llvm-svn: 243984