[RISCV] Match GNU tools canonical JALR and add aliases
The canonical GNU form of JALR resembles a load/store instruction rather
than placing the immediate offset as a separate argument, so match this
behaviour. Also add parser-only aliases for the three-operand form, and
add other shorter aliases also emitted by GNU tools.
Differential Revision: https://reviews.llvm.org/D55277
Patch by James Clarke.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366179
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