Patch is fixing assertion raised in case of copying 64-bit register into 32-bit one and vice versa. This assertion is raised in case when -mcpu=mips64r6 and -mattr=micromips are set for llc.
There are two test cases that are triggering this assertion. Test test/CodeGen/Mips/micromips-addiu.ll is for a case of copying 64-bit into 32-bit register and test test/CodeGen/Mips/micromips-gp-rc.ll is for a case of copying 32-bit into 64-bit register.
I don't think that we should add this comment to every test because it'll become confusing in the future as we add more RUN-lines. Could you pick a minimal test, or just one of the existing tests, and check this in a separate test-file?