This reverts commit 0f7f8f87747ae825c3b5adeae35cbb66114d2ba1, reversing
changes made to 9f1505435877cf8f5abe7b66fb2717c7e39c1988.

As it is currently written, CallLambdaTrampoline does not return a
value. However, some of the functions that are being wrapped may
return a value that the JIT is expected to understand. A compiler
*cough cough clang* may opt to alter %rax after the wrapped lambda
returns, e.g. popping a previous value, which can clobber the
return value. If we actually have a return value, then the compiler
must not clobber it.

Suggested by degasus.

Jit64: add FMA4 support to fmaddXX

Jit64: drop more x86_32 left-overs

Keep the more explicit version of the two SSECompare enums.

And the unused CMP* wrappers wouldn't exactly make the code more
readable anyway.

Considering OpArg is a struct, passing by value creates unnecessary copies.