[CodeGen][test] Use FileCheck variable matchers for better test support

Summary: Depending on how clang is built, it may discard the IR names and use names like `%2` instead of `%result.ptr`, causing tests that rely on the IR name to fail. Using FileCheck matchers makes the test work regardless of how clang is built.

This test passes with both `-fno-discard-value-names` and `-fdiscard-value-names` to make sure it passes regardless of the build mode.

Reviewers: rnk, akhuang, aprantl, lebedev.ri

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63625

llvm-svn: 363996