[AArch64][SVE] Instcombine SVE LD1/ST1 to stock LLVM IR

InstCombine AArch64 LD1/ST1 to llvm.masked.load/llvm.masked.store
and LD1/ST1 to load/store when a ptrue all predicate pattern operand
is present.

This allows existing IR optimizations such as dead-load removal to
occur.

Differential Revision: https://reviews.llvm.org/D113489