[ConstantRange] Fix a couple cases where we were possibly throwing away an APInt allocation we could reuse. NFC

This uses setAllBits to replace getMaxValue and operator=(uint64_t) instead of constructing an APInt from uint64_t.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301761 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed