[X86] Factor the core code out of LowerXALUO into a helper function. Use it in...

[X86] Factor the core code out of LowerXALUO into a helper function. Use it in LowerBRCOND and LowerSELECT to avoid some duplicated code.

This makes it easier to keep the LowerBRCOND and LowerSELECT code in sync with LowerXALUO so they always pick the same operation for overflowing instructions.

This is inspired by the helper functions used by ARM and AArch64 for the same purpose.

The test change is because LowerSELECT was not in sync with LowerXALUO with regard to INC/DEC for SADDO/SSUBO.

llvm-svn: 350198