cmd/compile: error if register is reused when setting edge state

When setting the edge state in register allocation we should only
be setting each register once. It is not possible for a register
to hold multiple values at once.

This CL converts the runtime error seen in #38195 into an internal
compiler error (ICE). It is better for the compiler to fail than
generate an incorrect program.

The bug reported in #38195 is now exposed as:

./parserc.go:459:11: internal compiler error: 'yaml_parser_parse_node': R5 is already set (v1074/v1241)

[stack trace]

Updates #38195.

Change-Id: Id95842fd850b95494cbd472b6fd5a55513ecacec
Reviewed-on: https://go-review.googlesource.com/c/go/+/228060


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>