AMDGPU/GlobalISel: Mark 32-bit G_ICMP as legal

[arsenm]

LGTM. Test name should be icmp?

lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp

AMDGPULegalizerInfo::AMDGPULegalizerInfo() {
   setAction({G_CONSTANT, S64}, Legal);
 
   setAction({G_FCONSTANT, S32}, Legal);
 
   setAction({G_GEP, P1}, Legal);
   setAction({G_GEP, P2}, Legal);
   setAction({G_GEP, 1, S64}, Legal);
 
+  setAction({G_ICMP, S1}, Legal);
+  setAction({G_ICMP, 1, S32}, Legal);
+
   setAction({G_LOAD, P1}, Legal);
   setAction({G_LOAD, P2}, Legal);
   setAction({G_LOAD, S32}, Legal);
   setAction({G_LOAD, 1, P1}, Legal);
   setAction({G_LOAD, 1, P2}, Legal);
 
   setAction({G_STORE, S32}, Legal);
   setAction({G_STORE, 1, P1}, Legal);

test/CodeGen/AMDGPU/GlobalISel/legalize-cmp.mir

+# RUN: llc -O0 -march=amdgcn -mcpu=fiji  -run-pass=legalizer -global-isel %s -o - | FileCheck %s
+
+--- |
+  define void @test_icmp() {
+  entry:
+    ret void
+  }
+...
+
+---
+name:            test_icmp
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+  - { id: 2, class: _ }
+body: |
+  bb.0.entry:
+    liveins: %vgpr0
+    %0(s32) = G_CONSTANT i32 0
+    %1(s32) = COPY %vgpr0
+
+    ; CHECK: %2(s1) = G_ICMP intpred(ne), %0(s32), %1
+    %2(s1) = G_ICMP intpred(ne), %0, %1
+...