[AArch64][GlobalISel] Fix regbankselect for G_FCMP with vector destinations

[paquette]

Summary

These should always go to a FPR, since they always use the vector registers.

[aemerson]

LGTM.

llvm/lib/Target/AArch64/GISel/AArch64RegisterBankInfo.cpp

case TargetOpcode::G_UITOFP: {
     break;
   }
   case TargetOpcode::G_FPTOSI:
   case TargetOpcode::G_FPTOUI:
     if (MRI.getType(MI.getOperand(0).getReg()).isVector())
       break;
     OpRegBankIdx = {PMI_FirstGPR, PMI_FirstFPR};
     break;
-  case TargetOpcode::G_FCMP:
-    OpRegBankIdx = {PMI_FirstGPR,
+  case TargetOpcode::G_FCMP: {
+    // If the result is a vector, it must use a FPR.
+    AArch64GenRegisterBankInfo::PartialMappingIdx Idx0 =
+        MRI.getType(MI.getOperand(0).getReg()).isVector() ? PMI_FirstFPR
+                                                          : PMI_FirstGPR;
+    OpRegBankIdx = {Idx0,
                     /* Predicate */ PMI_None, PMI_FirstFPR, PMI_FirstFPR};
     break;
+  }
   case TargetOpcode::G_BITCAST:
     // This is going to be a cross register bank copy and this is expensive.
     if (OpRegBankIdx[0] != OpRegBankIdx[1])
       Cost = copyCost(
           *AArch64GenRegisterBankInfo::PartMappings[OpRegBankIdx[0]].RegBank,
           *AArch64GenRegisterBankInfo::PartMappings[OpRegBankIdx[1]].RegBank,
           OpSize[0]);
     break;

llvm/test/CodeGen/AArch64/GlobalISel/regbank-fcmp.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64 -run-pass=regbankselect -verify-machineinstrs %s -o - | FileCheck %s
+
+...
+---
+name:            vector
+legalized: true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+  liveins: $q0, $q1
+    ; Vectors should always end up on a FPR.
+
+    ; CHECK-LABEL: name: vector
+    ; CHECK: liveins: $q0, $q1
+    ; CHECK: %x:fpr(<2 x s64>) = COPY $q0
+    ; CHECK: %y:fpr(<2 x s64>) = COPY $q1
+    ; CHECK: %fcmp:fpr(<2 x s64>) = G_FCMP floatpred(olt), %x(<2 x s64>), %y
+    ; CHECK: $q0 = COPY %fcmp(<2 x s64>)
+    ; CHECK: RET_ReallyLR implicit $q0
+    %x:_(<2 x s64>) = COPY $q0
+    %y:_(<2 x s64>) = COPY $q1
+    %fcmp:_(<2 x s64>) = G_FCMP floatpred(olt), %x:_(<2 x s64>), %y:_
+    $q0 = COPY %fcmp
+    RET_ReallyLR implicit $q0
+...