[dev.unified] cmd/compile: visit LHS before RHS/X in assign/for statement

[dev.unified] cmd/compile: visit LHS before RHS/X in assign/for statement

Unified IR used to visit RHS/X before LHS in assign/for statements for
satisfying toolstash in quirksmode.

After CL 385998, unified IR quirks mode was gone, the constraint to
visit RHS/X first is no longer necessary.

Change-Id: I1c3825168b67fb094928f5aa21748a3c81b118ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/410343
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>