Bitset: remove some GCC<6.2 workarounds related to bitwise operators

GCC<6.2 has been unsupported since April 2022 (commit 4c72deb6).

X86TargetParser.cpp has another workaround that the other 2 nearly identical places don't have. Remove them as well.

Reviewed By: arsenm, craig.topper

Differential Revision: https://reviews.llvm.org/D158687