cmd/compile: implement signed loads from read-only memory

cmd/compile: implement signed loads from read-only memory

In addition to unsigned loads which already exist.

This helps code that does switches on strings to constant-fold
the switch away when the string being switched on is constant.

Fixes #71699

Change-Id: If3051af0f7255d2a573da6f96b153a987a7f159d
Reviewed-on: https://go-review.googlesource.com/c/go/+/649295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>