Merge commit 95e18b2d9d5f from llvm git (by Kang Zhang):

[PowerPC] Fix a typo for InstAlias of mfsprg

D77531 has a type for mfsprg, it should be mtsprg. This patch is to
fix this typo.

This should fix booting powerpc64 kernels, after LLVM 11 was imported.

PR: 248763
(cherry picked from commit 2a99bea5784f4b347a70967fe4b8faf1c7280f79)