[RISCV][GISel] Mark G_GLOBAL_VALUE as legal.

[reames]

LGTM

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

getActionDefinitionsBuilder(G_BRCOND)
       .legalFor({XLenLLT})
       .minScalar(0, XLenLLT);
 
   getActionDefinitionsBuilder(G_PHI)
       .legalFor({p0, XLenLLT})
       .widenScalarToNextPow2(0)
       .clampScalar(0, XLenLLT, XLenLLT);
 
+  getActionDefinitionsBuilder(G_GLOBAL_VALUE)
+      .legalFor({p0});
+
   getLegacyLegalizerInfo().computeTables();
 }

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

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=riscv32 -run-pass=legalizer %s -o - | FileCheck %s
+
+--- |
+  @var = global i32 0
+
+  define ptr @test_global() {
+    ret ptr @var
+  }
+
+...
+---
+name:            test_global
+alignment:       4
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: _ }
+frameInfo:
+  maxAlignment:    1
+machineFunctionInfo:
+  varArgsFrameIndex: 0
+  varArgsSaveSize: 0
+body:             |
+  bb.1 (%ir-block.0):
+    ; CHECK-LABEL: name: test_global
+    ; CHECK: [[GV:%[0-9]+]]:_(p0) = G_GLOBAL_VALUE @var
+    ; CHECK-NEXT: $x10 = COPY [[GV]](p0)
+    ; CHECK-NEXT: PseudoRET implicit $x10
+    %0:_(p0) = G_GLOBAL_VALUE @var
+    $x10 = COPY %0(p0)
+    PseudoRET implicit $x10
+
+...

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

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=riscv64 -run-pass=legalizer %s -o - | FileCheck %s
+
+--- |
+  @var = global i32 0
+
+  define ptr @test_global() {
+    ret ptr @var
+  }
+
+...
+---
+name:            test_global
+alignment:       4
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: _ }
+frameInfo:
+  maxAlignment:    1
+machineFunctionInfo:
+  varArgsFrameIndex: 0
+  varArgsSaveSize: 0
+body:             |
+  bb.1 (%ir-block.0):
+    ; CHECK-LABEL: name: test_global
+    ; CHECK: [[GV:%[0-9]+]]:_(p0) = G_GLOBAL_VALUE @var
+    ; CHECK-NEXT: $x10 = COPY [[GV]](p0)
+    ; CHECK-NEXT: PseudoRET implicit $x10
+    %0:_(p0) = G_GLOBAL_VALUE @var
+    $x10 = COPY %0(p0)
+    PseudoRET implicit $x10
+
+...