[AArch64] Wrap integer SCALAR_TO_VECTOR nodes in bitcasts (#172837)
This patch changes lowering of SCALAR_TO_VECTOR node with i32 or i64 by
inserting bitconvert to/from floating point type infront of its
operands. This bitconverts serve as a hint to instruction selector and
they make it clear that the operand and return value of this node need
to be NEON registers.
Depends on: #179315
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Co-authored-by: Kerry McLaughlin <kerry.mclaughlin@arm.com>