Merge commit 4d52ebb9b9c7 from llvm git (by Chen Zheng):

[PowerPC] Make StartMI ignore COPY like instructions.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D85659

This fixes an assertion failure when building world for powerpc. It was
reported upstream as https://bugs.llvm.org/show_bug.cgi?id=47041.