[InstCombine] re-use bitcasted compare operands in selects (PR28001)

These mixed bitcast patterns show up with SSE/AVX intrinsics because we bitcast function parameters to <2 x i64>.

The bitcasts obfuscate the expected min/max forms as shown in PR28001:
https://llvm.org/bugs/show_bug.cgi?id=28001#c6

Differential Revision: https://reviews.llvm.org/D25943

llvm-svn: 285495