[WebAssembly] Use DebugValueManager only when subprogram exists (#77978)

We previously scanned the whole BB for `DBG_VALUE` instruction even when
the program doesn't have debug info, i.e., the function doesn't have a
subprogram associated with it, which can make compilation unnecessarily
slow. This disables `DebugValueManager` when a `DISubprogram` doesn't
exist for a function.

This only reduces unnecessary work in non-debug mode and does not change
output, so it's hard to add a test to test this behavior.

Test changes were necessary because their `DISubprogram`s were not
correctly linked with the functions, so with this PR the compiler
incorrectly assumed the functions didn't have a subprogram and the tests
started to fail.

Fixes https://github.com/emscripten-core/emscripten/issues/21048.