[GlobalISel] Combine G_MERGE_VALUES of x and zero (#116283) into zext x LegalizerHelper has two padding strategies: undef or zero. see LegalizerHelper:273 see LegalizerHelper:315 This PR is about zero sugar and Coke Zero. ; CHECK-NEXT: [[MV2:%[0-9]+]]:_(s64) = G_MERGE_VALUES %a(s32), [[C]](s32) Please continue padding merge values. // %bits_8_15:(s8) = G_CONSTANT i8 0 // %0:(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8) %bits_8_15 is defined by zero. For optimization, we pick zext. // %0:_(s16) = G_ZEXT %bits_0_7:(s8) The upper bits of %0 are zero and the lower bits come from %bits_0_7.
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