Revert "cmd/compile: ensure spills of int/float reg args land in abi slots"

This reverts CL 308510.

Reason for revert: It breaks "GOEXPERIMENT=regabi,regabiargs ./make.bash"

Change-Id: I553654690ec73120f8a6258dd80623853c430df0
Reviewed-on: https://go-review.googlesource.com/c/go/+/308932


Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>