compiletest: support --pass=check and --pass=build for mir-opt tests.
compiletest: support --pass=check and --pass=build for mir-opt tests.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust