[ARM] Create VMOVRRD from adjacent vector extracts
This adds a combine for extract(x, n); extract(x, n+1) -> VMOVRRD(extract x, n/2). This allows two vector lanes to be moved at the same time in a single instruction, and thanks to the other VMOVRRD folds we have added recently can help reduce the amount of executed instructions. Floating point types are very similar, but will include a bitcast to an integer type. This also adds a shouldRewriteCopySrc, to prevent copy propagation from DPR to SPR, which can break as not all DPR regs can be extracted from directly. Otherwise the machine verifier is unhappy. Differential Revision: https://reviews.llvm.org/D100244
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