[AArch64] Improve the cost model for extending mull (#125651) We already have cost model code for detecting extending mull multiplies for the form `mul(ext, ext)`. Since it was added the codegen for mull has been improved, this attempts to catch the cost model up. The main idea is to incorporate extends of larger sizes. A vector `v8i32 mul(zext(v8i8), zext(v8i8))` will be code-generated as `zext (v8i16 mul(zext(v8i8), zext(v8i8))`, or umull+ushll+ushll2. So the total cost should be 3ish if each instruction costs 1. Where exactly we attribute the costs is dependable, this patch opts to sets the cost of the extend to 0 (or the cost of the extend not included in the mull) and the mul gets the cost of the mull+extra extends. isWideningInstruction is split into two functions for the two types of operands it supports. isSingleExtWideningInstruction now handles addw instructions that extend the second operand, isBinExtWideningInstruction is for instructions like addl that extend both operands.
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