compiler: discard global sink variables with static initializers

compiler: discard global sink variables with static initializers

This is specifically for the test fixedbugs/issue23781.go, which
builds a large static array.  The code does compile and work without
this change, but it takes a long time and generates a large object
file.  Discarding the unnecessary static initializer makes this test
much faster.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/278172