[X86] Use inlineasm flag output for the _bittest* intrinsics.

Instead of expliciting emitting a setc in the inline asm instructions,
we can use flag output. This allows the backend to use the flag
directly if it is needed by a branch. Previously we needed a test
instruction to convert the register back to a flag.

If the flag can't be used directly, the backend will emit a setcc.

Differential Revision: https://reviews.llvm.org/D87888