compiler: add Ptrmask_symbol_expression

Add an expression to evaluate to the ptrmask for a type.  This will be
used for global variables, which always use a ptrmask no matter how
large they are.

Change-Id: I0b79a31f4280d44f9ffbaf9fda1b265b384028c3
Reviewed-on: https://go-review.googlesource.com/40981
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed