[x86] add tests for maxnum/minnum intrinsics with nnan; NFC

Clang 6.0 was updated to create these intrinsics rather than 
libcalls or fcmp/select, but the backend wasn't prepared to 
handle that optimally. 

This bug is not the primary reason for PR37403:
https://bugs.llvm.org/show_bug.cgi?id=37403
...but it's probably more important for x86 perf.

llvm-svn: 331988