[InstCombine] exclude bitcast of ppc_fp128 in icmp signbit fold

Based on the post-commit comments for rG0f56bbc, there might
be a problem with this transform:

(bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0

...and the ppc_fp128 data type, so conservatively bypass if we
are bitcasting a ppc_fp128.

We might be able to account for endian or other differences to
enable this for PowerPC again if that is useful.

Differential Revision: https://reviews.llvm.org/D77642