Fix typo in risc-v register aliases.
Fix typo in risc-v register aliases.

Patch by John.

Differential Revision: https://reviews.llvm.org/D61464

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@360104 91177308-0d34-0410-b5e6-96231b3b80d8

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