cmd/internal/obj/wasm: refactor handling of wasm variables

This commit improves how registers get mapped to wasm variables. This
is a preparation for future improvements (e.g. adding 32 bit float
registers).

Change-Id: I374c80b2d6c9bcce6b0e373fe921b5ad4dee40ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/191777
Run-TryBot: Richard Musiol <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: neelance

src/cmd/internal/obj/wasm/a.out.go

@@ -48,6 +48,12 @@ const (
 	ADrop // opcode 0x1A
 	ASelect
 
+	ALocalGet // opcode 0x20
+	ALocalSet
+	ALocalTee
+	AGlobalGet
+	AGlobalSet
+
 	AI32Load // opcode 0x28
 	AI64Load
 	AF32Load