cmd/compile: pre init shapeMap
cmd/compile: pre init shapeMap

Change-Id: I4fea927b9d34e7bf382ddf148a787c1558176f2f
GitHub-Last-Rev: ff1fa78a3601243092f84a69e7332ec2ba9ec05a
GitHub-Pull-Request: golang/go#50568
Reviewed-on: https://go-review.googlesource.com/c/go/+/377994
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go