[dev.inline] cmd/compile/internal/ssa: another round of renames from line -> pos (cleanup)

Mostly mechanical renames. Make variable names consistent with use.

Change-Id: Iaa89d31deab11eca6e784595b58e779ad525c8a3
Reviewed-on: https://go-review.googlesource.com/34146


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>