compiler: add explicit convert in Type_guard_expression::do_get_backend

The current recipe in Type_guard_expression for creating a Bexpression
performs a type conversion at the Type level, but doesn't invoke
Backend::convert_expression to capture the conversion for the back
end. Add code to create a BE type conversion operation.

Change-Id: I395b3ab9e8aa2c1413055f8df3d9a87b5a0d4ffa
Reviewed-on: https://go-review.googlesource.com/50373
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed