[dev.ssa] cmd/compile: reduce line number churn in generated code

[dev.ssa] cmd/compile: reduce line number churn in generated code

In regalloc, make LoadReg instructions use the line number
of their *use*, not their *source*.  This reduces the
tendency of debugger stepping to "jump around" the program.

Change-Id: I59e2eeac4dca9168d8af3a93effbc5bdacac2881
Reviewed-on: https://go-review.googlesource.com/19836
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>