[HLSL] Make lerp overload tests stricter. NFC
These tests had checks across multiple functions, because none of the hlsl
header implementation functions were inlined. Switch the run lines to use -O1
instead of -disable-llvm-passes and rewrite the tests to actually look at the
whole function.
Note - this would be simpler with update_cc_test_checks, but the differences
between DX and SPIRV are difficult to account for with those scripts.
