[AArch64][SVE][InstCombine] Move last{a,b} before binop if one operand is a splat value

Move the last{a,b} operation to the vector operand of the binary instruction if
the binop's operand is a splat value. This essentially converts the binop
to a scalar operation.

Example:
    // If x and/or y is a splat value:
    lastX (binop (x, y)) --> binop(lastX(x), lastX(y))

Differential Revision: https://reviews.llvm.org/D106932

Change-Id: I93ff5302f9a7972405ee0d3854cf115f072e99c0