[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