[WebAssembly] Fix argument types in SIMD narrowing intrinsics

The builtins were updated to take signed parameters in 627a5269, but the
intrinsics that use those builtins were not updated as well. The intrinsic test
did not catch this sign mismatch because it is only reported as an error under
-fno-lax-vector-conversions.

This commit fixes the type mismatch and adds -fno-lax-vector-conversions to the
test to catch similar problems in the future.

Differential Revision: https://reviews.llvm.org/D101979