cmd/compile: try to preserve IsStmt marks from OpConvert

Note when a statement mark was not consumed during Prog
generation, and try to use it on a subsequent opcode so
that the statement marker will not be lost.

And a test.

Fixes #49628.

Change-Id: I03f7782a9809cc4a0a5870df92b3e182cf124554
Reviewed-on: https://go-review.googlesource.com/c/go/+/366694


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>