[CodeGen][inlineasm] assume the flag output of inline asm is boolean value

GCC inline asm document says that
"... the general rule is that the output variable must be a scalar
integer, and the value is boolean."

Commit e5c37958 lowers flag output of
inline asm on X86 with setcc, hence it is guaranteed that the flag
is of boolean value. Clang does not support ARM inline asm flag output
yet so nothing need to be worried about ARM.

See "Flag Output" section at
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#OutputOperands

Fixes https://github.com/llvm/llvm-project/issues/56568

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D129954