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Implement AArch64 variant of shouldCoalesce() to detect a known failing case and prevent the coalescing of a 32-bit copy into a 64-bit sign-extending load. Do not coalesce in the following case: COPY where source is bottom 32 bits of a 64-register, and destination is a 32-bit subregister of a 64-bit register, ie it causes the rest of the register to be implicitly set to zero. A mir test has been added. In the test case, the 32-bit copy implements a 32 to 64 bit zero extension and relies on the upper 32 bits being zeroed. Coalescing to the result of the 64-bit load meant overwriting the upper 32 bits incorrectly when the loaded byte was negative. Reviewed By: john.brawn Differential Revision: https://reviews.llvm.org/D85956
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