[mips] Add a pattern for 64-bit GPR variant of the `rdhwr` instruction

MIPS ISAs start to support third operand for the `rdhwr` instruction
starting from Revision 6. But LLVM generates assembler code with
three-operands version of this instruction on any MIPS64 ISA. The third
operand is always zero, so in case of direct code generation we get
correct code.

This patch fixes the bug by adding an instruction alias. The same alias
already exists for 32-bit ISA.

Ideally, we also need to reject three-operands version of the `rdhwr`
instruction in an assembler code if ISA revision is less than 6. That is
a task for a separate patch.

This fixes PR38861 (https://bugs.llvm.org/show_bug.cgi?id=38861)

Differential revision: https://reviews.llvm.org/D51773

llvm-svn: 341919