[SVE][LSR] Teach LSR to enable simple scaled-index addressing mode generation for SVE.

Currently, Loop strengh reduce is not handling loops with scalable stride very well.

Take loop vectorized with scalable vector type <vscale x 8 x i16> for instance,
(refer to test/CodeGen/AArch64/sve-lsr-scaled-index-addressing-mode.ll added).

Memory accesses are incremented by "16*vscale", while induction variable is incremented
by "8*vscale". The scaling factor "2" needs to be extracted to build candidate formula
i.e., "reg(%in) + 2*reg({0,+,(8 * %vscale)}". So that addrec register reg({0,+,(8*vscale)})
can be reused among Address and ICmpZero LSRUses to enable optimal solution selection.

This patch allow LSR getExactSDiv to recognize special cases like "C1*X*Y /s C2*X*Y",
and pull out "C1 /s C2" as scaling factor whenever possible. Without this change, LSR
is missing candidate formula with proper scaled factor to leverage target scaled-index
addressing mode.

Note: This patch doesn't fully fix AArch64 isLegalAddressingMode for scalable
vector. But allow simple valid scale to pass through.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D103939