[X86] Use bts/btr/btc for single bit set/clear/complement of a variable bit position

If we are just modifying a single bit at a variable bit position we can use the BT* instructions to make the change instead of shifting a 1(or rotating a -1) and doing a binop. These instruction also ignore the upper bits of their index input so we can also remove an and if one is present on the index.

Fixes PR37938.

llvm-svn: 335754