[RISCV][GISel] Legalize G_{U|S}{ADD|SUB}SAT (#92935)

This patch adds support for G_UADDSAT/G_SADDSAT/G_USUBSAT/G_SSUBSAT by
lowering it into add/sub with selects. When zbb is available,
min/max/minu/maxu will be selected.

Author: dtcxzyw

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp

@@ -130,6 +130,10 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST)
 
   getActionDefinitionsBuilder({G_SADDO, G_SSUBO}).minScalar(0, sXLen).lower();
 
+  // TODO: Use Vector Single-Width Saturating Instructions for vector types.
+  getActionDefinitionsBuilder({G_UADDSAT, G_SADDSAT, G_USUBSAT, G_SSUBSAT})
+      .lower();
+
   auto &ShiftActions = getActionDefinitionsBuilder({G_ASHR, G_LSHR, G_SHL});
   if (ST.is64Bit())
     ShiftActions.customFor({{s32, s32}});