AArch64: look up EmitAArch64Scalar support before calling.

This fixes one immediate bug where an expression with side-effects
could be emitted twice during a NEON call.

It also prepares the way for folding CodeGen for many of the SISD
intrinsics into a table, reducing code size and hopefully increasing
performance eventually ("binary search + few switch cases" should be
better than "lots of switch cases").