Revert "[AArch64][GlobalISel] Lower formal arguments of AAPCS & ms_abi variadic functions."

This reverts commit 56fd846f.

This commit regressed handling of functions with floats as arguments,
reproducible e.g. like this:

$ cat test.c
double func(double f) {
    return f;
}
$ clang -target aarch64-windows -S -o - test.c -fno-asynchronous-unwind-tables
func:
	sub	sp, sp, #16
	str	x0, [sp, #8]
	ldr	d0, [sp, #8]
	add	sp, sp, #16
	ret