Exclude bitcast and ext/trunc signbit optimization on ppc_fp128

Revision a1c05fe2 <https://reviews.llvm.org/rGa1c05fe20f3def1f1be9f50d2adefc6b6f1578ad>
removed bitcast from the list of problematic transformations, however:

  %97 = fptrunc ppc_fp128 %2 to double            // we need to check ppc_fp128 here to prevent the transformation
  %98 = bitcast double %97 to i64                 // a1c05fe2 checks ppc_fp128 at here
  %99 = icmp slt i64 %98, 0
  %100 = zext i1 %99 to i8
  store i8 %100, i8* %7, align 1

so this patch does that. I'm also disabling it in the presence of extend just in case.

I verified separately that the hash of -std::infinity and std::infinity don't match now.

Differential Revision: https://reviews.llvm.org/D77911