[AArch64][GlobalISel] Improve divide by constant

[davemgreen]

SDAG has some improved code generation for expanding sdiv/udiv by a constant, avoiding the need for expensive sdiv/udiv operations and scalarization of vector types. We should look into improving the same capability in GISel.

Scalar: https://godbolt.org/z/W817YfWxW
Vector: https://godbolt.org/z/c9ME7Eqc4

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: David Green (davemgreen)

SDAG has some improved code generation for expanding sdiv/udiv by a constant, avoiding the need for expensive sdiv/udic operations and scalarization of vector types. We should look into improving the same capability in GISel.

Scalar: https://godbolt.org/z/W817YfWxW
Vector: https://godbolt.org/z/c9ME7Eqc4