cmd/compile: preserve statement mark in rematerialized values

Statement markers on rematerializable values were getting lost in
register allocation.  This checks for that case (rematerializable
input and using value share line number, but mark is on the input)
and preserves the mark.

When combined with other CLs in this series, this CL reduces the
"nostmt" count (a line appears in the assembly, but no statement
marker) for cmd/go from 413 to 277.  The rematerialized input is
usually a LEAQ (on AMD64).

The cause is "complicated"; for example, a NilCheck originally has the
statement mark (a good thing, if the NilCheck  remains) but the
NilCheck is removed and the mark floats to a Block end, then to a
SliceMake.  The SliceMake decomposes and goes dead without preserving
its marker (its component values are elided in other rewrites and may
target inputs with different line numbers), but before deadcode
removes it from the graph it moves the mark to an input, which at that
time happens to be a LocalAddr. This eventually transforms to a LEAQ.

Change-Id: Iff91fc2a934357fb59ec46ac87b4a9b1057d9160
Reviewed-on: https://go-review.googlesource.com/c/go/+/198480


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>