[dev.unified] cmd/compile: rename haveRType and implicitExpr

[dev.unified] cmd/compile: rename haveRType and implicitExpr

This CL renames:

1. "haveRType" to "hasRType", suggested by drchase@ during review of
CL 413358; and

2. "implicitExpr" to "implicitConvExpr", suggested by khr@ during
review of CL 413396.

Change-Id: Ibb4deae20908d960706640991ea44d1b9c0b9e3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/413854
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>