[x86] add more tests for select-of-constants; NFC

This is to help recommit a fixed version of r310208. As shown in PR34097, 
we could miscompile if subtraction of the constants overflowed.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310490 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed