Merge r341919 into the 7.0 branch: [mips] Add a pattern for 64-bit GPR variant of the `rdhwr` instruction

[atanasyan]

Bugzilla Link	39611
Resolution	FIXED
Resolved on	Nov 12, 2018 21:43
Version	trunk
OS	All
Blocks	#38454
CC	@tstellar
Fixed by commit(s)	r341919 r346739

Extended Description

Please merge the following revision to the 7.0 branch. This fixes #38209 and restores building LLVM/Clang by MIPS GCC under some conditions.

https://reviews.llvm.org/rL341919

[tstellar]

Merged: r346739