[HLSL] Make lerp overload tests stricter. NFC (#137898)
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.
