cmd/compile: enforce 32-bit restrictions on ops

Most 64-bit x86 ops can only take a signed 32-bit constant.
Clean up our rewrite rules to enforce this restriction.

Modify the assembler to fail if the offset does not fit
in the instruction.

That last check triggers a few times on weird testing code.
Suppress those errors if the compiler itself generated errors.

Fixes #14862

Change-Id: I76559af035b38483b1e59621a8029fc66b3a5d1e
Reviewed-on: https://go-review.googlesource.com/20815
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/gc/dcl.go

@@ -1309,6 +1309,10 @@ func funccompile(n *Node) {
 	breakpc = nil
 	Funcdepth = 0
 	dclcontext = PEXTERN
+	if nerrors != 0 {
+		// If we have compile errors, ignore any assembler/linker errors.
+		Ctxt.DiagFunc = func(string, ...interface{}) {}
+	}
 	flushdata()
 	obj.Flushplist(Ctxt) // convert from Prog list to machine code
 }