[dev.inline] cmd/compile/internal/ssa: rename various fields from Line to Pos

This is a mostly mechanical rename followed by manual fixes where necessary.

Change-Id: Ie5c670b133db978f15dc03e50dc2da0c80fc8842
Reviewed-on: https://go-review.googlesource.com/34137


Reviewed-by: David Lazar <lazard@golang.org>