[AArch64][GlobalISel] Mark G_BR as always legal. NFC (#153545)

As far as I understand this will not affect anything, just lower the
exposure to the legacy legalizer rules.

Author: davemgreen

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

@@ -968,6 +968,7 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
                    {s128, s64}});
 
   // Control-flow
+  getActionDefinitionsBuilder(G_BR).alwaysLegal();
   getActionDefinitionsBuilder(G_BRCOND)
     .legalFor({s32})
     .clampScalar(0, s32, s32);

llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir

@@ -674,8 +674,8 @@
 # DEBUG-NEXT: .. the first uncovered type index: 2, OK
 # DEBUG-NEXT: .. the first uncovered imm index: 0, OK
 # DEBUG-NEXT: G_BR (opcode {{[0-9]+}}): 0 type indices, 0 imm indices
-# DEBUG-NEXT: .. type index coverage check SKIPPED: no rules defined
-# DEBUG-NEXT: .. imm index coverage check SKIPPED: no rules defined
+# DEBUG-NEXT: .. type index coverage check SKIPPED: user-defined predicate detected
+# DEBUG-NEXT: .. imm index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: G_BRJT (opcode {{[0-9]+}}): 2 type indices
 # DEBUG-NEXT: .. the first uncovered type index: 2, OK
 # DEBUG-NEXT: .. the first uncovered imm index: 0, OK