[RISCV][GISel] Use libcalls for G_MEMCPY, G_MEMMOVE, and G_MEMSET.

Author: topperc

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

@@ -189,6 +189,8 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST) {
 
   getActionDefinitionsBuilder(G_FRAME_INDEX).legalFor({p0});
 
+  getActionDefinitionsBuilder({G_MEMCPY, G_MEMMOVE, G_MEMSET}).libcall();
+
   // FP Operations
 
   getActionDefinitionsBuilder({G_FADD, G_FSUB, G_FMUL, G_FDIV, G_FMA, G_FNEG,