AArch64: Try to move 32-bit stores together for pairing

[david-arm]

We sometimes see sequences of vasm operations like this:

ldimml ..., %x storel %x, [%sp - 0x80] ldimml ..., %y storel %y, [%sp - 0x7c]

The stores are actually contiguous in memory and if they were next to each other they would be paired up into a storepairl. This patch looks for such patterns and rewrites them as

ldimml ..., %x ldimml ..., %y storepairl %x, %y, [%sp - 0x80]

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