[IRBuilder] fix CreateMaxNum to actually produce maxnum (PR36454)

The bug was introduced here:
https://reviews.llvm.org/rL296409
...but the patch doesn't use maxnum and nothing else in 
trunk has tried since then, so the bug went unnoticed.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325607 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed