[AArch64][GlobalISel] Expand abs.v4i8 to v4i16 and abs.v2s16 to v2s32 (#81231)

GISel was currently falling back to SDAG for these functions, and this
matches the way SDAG currently generates code for these functions.

Author: dc03-work

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

@@ -1009,6 +1009,12 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
     ABSActions
         .legalFor({s32, s64});
   ABSActions.legalFor(PackedVectorAllTypeList)
+      .widenScalarIf(
+          [=](const LegalityQuery &Query) { return Query.Types[0] == v4s8; },
+          [=](const LegalityQuery &Query) { return std::make_pair(0, v4s16); })
+      .widenScalarIf(
+          [=](const LegalityQuery &Query) { return Query.Types[0] == v2s16; },
+          [=](const LegalityQuery &Query) { return std::make_pair(0, v2s32); })
       .clampNumElements(0, v8s8, v16s8)
       .clampNumElements(0, v4s16, v8s16)
       .clampNumElements(0, v2s32, v4s32)

llvm/test/CodeGen/AArch64/abs.ll

@@ -2,9 +2,6 @@
 ; RUN: llc -mtriple=aarch64-none-linux-gnu %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-SD
 ; RUN: llc -mtriple=aarch64-none-linux-gnu -global-isel -global-isel-abort=2 %s -o - 2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-GI
 
-; CHECK-GI:         warning: Instruction selection used fallback path for abs_v4i8
-; CHECK-GI-NEXT:    warning: Instruction selection used fallback path for abs_v2i16
-
 ; ===== Legal Scalars =====
 
 define i8 @abs_i8(i8 %a){