[RISCV][GISel] Make G_CONSTANT of pointers legal.

[craig.topper]

Summary

This is needed to support things like null pointers.

[reames]

LGTM

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

 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Type.h"
 
 using namespace llvm;
 
 RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST) {
   const unsigned XLen = ST.getXLen();
   const LLT XLenLLT = LLT::scalar(XLen);
+  const LLT p0 = LLT::pointer(0, XLen);
 
   using namespace TargetOpcode;
 
   getActionDefinitionsBuilder({G_ADD, G_SUB, G_AND, G_OR, G_XOR})
       .legalFor({XLenLLT})
       .widenScalarToNextPow2(0)
       .clampScalar(0, XLenLLT, XLenLLT);
 
for (unsigned Op : {G_MERGE_VALUES, G_UNMERGE_VALUES}) {
     getActionDefinitionsBuilder(Op)
         .widenScalarToNextPow2(LitTyIdx, XLen)
         .widenScalarToNextPow2(BigTyIdx, XLen)
         .clampScalar(LitTyIdx, XLenLLT, XLenLLT)
         .clampScalar(BigTyIdx, XLenLLT, XLenLLT);
   }
 
   getActionDefinitionsBuilder({G_CONSTANT, G_IMPLICIT_DEF})
-      .legalFor({XLenLLT})
+      .legalFor({XLenLLT, p0})
       .widenScalarToNextPow2(0)
       .clampScalar(0, XLenLLT, XLenLLT);
 
   getActionDefinitionsBuilder(G_ICMP)
       .legalFor({{XLenLLT, XLenLLT}})
       .widenScalarToNextPow2(1)
       .clampScalar(1, XLenLLT, XLenLLT)
       .clampScalar(0, XLenLLT, XLenLLT);

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/rv32/legalize-const.mir

bb.0.entry:
     %0:_(s96) = G_CONSTANT i96 -214748364921
     %1:_(s32), %2:_(s32), %3:_(s32) = G_UNMERGE_VALUES %0(s96)
     $x10 = COPY %1(s32)
     $x11 = COPY %2(s32)
     $x12 = COPY %3(s32)
     PseudoRET implicit $x10, implicit $x11, implicit $x12
 
 ...
+---
+name:            const_null
+body:             |
+  bb.0.entry:
+    ; CHECK-LABEL: name: const_null
+    ; CHECK: [[C:%[0-9]+]]:_(p0) = G_CONSTANT i32 0
+    ; CHECK-NEXT: $x10 = COPY [[C]](p0)
+    ; CHECK-NEXT: PseudoRET implicit $x10
+    %0:_(p0) = G_CONSTANT i32 0
+    $x10 = COPY %0(p0)
+    PseudoRET implicit $x10
+
+...

llvm/test/CodeGen/RISCV/GlobalISel/legalizer/rv64/legalize-const.mir

bb.0.entry:
     %0:_(s192) = G_CONSTANT i192 -214772364921
     %1:_(s64), %2:_(s64), %3:_(s64) = G_UNMERGE_VALUES %0(s192)
     $x10 = COPY %1(s64)
     $x11 = COPY %2(s64)
     $x12 = COPY %3(s64)
     PseudoRET implicit $x10, implicit $x11, implicit $x12
 
 ...
+---
+name:            const_null
+body:             |
+  bb.0.entry:
+    ; CHECK-LABEL: name: const_null
+    ; CHECK: [[C:%[0-9]+]]:_(p0) = G_CONSTANT i64 0
+    ; CHECK-NEXT: $x10 = COPY [[C]](p0)
+    ; CHECK-NEXT: PseudoRET implicit $x10
+    %0:_(p0) = G_CONSTANT i64 0
+    $x10 = COPY %0(p0)
+    PseudoRET implicit $x10
+
+...