Revert "cmd/compile: make wasm match other platforms for FP->int32/64 conversions"

Revert "cmd/compile: make wasm match other platforms for FP->int32/64 conversions"

This reverts commit 8d810286b3121b601480426159c04d178fa29166.

Reason for revert: we need to do this more carefully, at minimum gated by a module version

Change-Id: Ia951e2e5ecdd455ea0f17567963c6fab0f4540dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/711840
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>