Merge r341221 into the 7.0 branch: [mips] Fix `mtc1` and `mfc1` definitions for microMIPS R6

[atanasyan]

Bugzilla Link	39610
Resolution	FIXED
Resolved on	Nov 12, 2018 21:28
Version	trunk
OS	All
Blocks	#38454
CC	@tstellar
Fixed by commit(s)	r341221 r346737

Extended Description

Please merge the following revision to the 7.0 branch. The fix was intended for release in 7.0 but missed the deadline.

https://reviews.llvm.org/rL341221

[tstellar]

Merged: r346737