[InstCombine] Use Twines to generate names.

Since the names are used in a loop this does more work in debug builds. In
release builds value names are generally discarded so we don't have to do
the concatenation at all. It's also simpler code, no functional change
intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263215 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed