rename instcombine test file. NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298904 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Transforms/InstCombine/phi-select-constexpr.ll b/test/Transforms/InstCombine/phi-select-constant.ll
similarity index 100%
rename from test/Transforms/InstCombine/phi-select-constexpr.ll
rename to test/Transforms/InstCombine/phi-select-constant.ll