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