[AArch64][GlobalISel] Fix nonterminating legalization for <8 x s4> vectors. (#192747) G_CONCAT_VECTORS with <16 x s4> sources hits the bitcast legalization path, which round-trips through scalar types (e.g. s32) and regenerates <8 x s4> vectors via G_UNMERGE_VALUES and G_BUILD_VECTOR. The G_BUILD_VECTOR is then widened to <8 x s8> (via .minScalarOrElt(0, s8)), producing G_ANYEXT/G_TRUNC artifact pairs. The artifact combiner folds these pairs away, restoring the original <8 x s4> types, which feeds back into G_CONCAT_VECTORS again. This change: * Adds .minScalarOrElt(1, s8) to the G_ICMP rules to ensure operand vector elements are at least s8. This causes <16 x s4> operands to be widened to <16 x s8>, and the result type follows via minScalarEltSameAs. * Add custom legalization for G_CONCAT_VECTORS when element size < 8. The custom handler widens source operands via G_ANYEXT (e.g. <8 x s4> -> <8 x s8>), concats the widened vectors (producing a legal type like <16 x s8>), and truncs the result back to the original type. The code is bad but we can fix that later, some fallbacks are fixed with this. rdar://153760145
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