[Patch] [AArch64] Fix registerAllocator assigns same register for base and wback in pre/post-index load and store

[Steven Wu]

Hi

This patch fix a bug that sometimes register allocator may assign same register as base and writeback register in pre/post-index loads and stores for AArch64.
According to ARMv8 manual, such result will have uncertain behaviors and clang as will reject them as well. Adding earlyclobber constraints to avoid them.
Since this behavior will depend on the register allocator so I am not sure how to produce a test. I can provide a testcase if anyone want to reproduce the problem.

Patch attached. Please review and commit it for me.

Thanks

Steven
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