[GlobalISel] Bail out early for big-endian

If we continue through the function we can currently hit crashes. We can bail
out early and fall back to SDAG.

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -3889,6 +3889,7 @@ bool IRTranslator::runOnMachineFunction(MachineFunction &CurMF) {
                                F.getSubprogram(), &F.getEntryBlock());
     R << "unable to translate in big endian mode";
     reportTranslationError(*MF, *TPC, *ORE, R);
+    return false;
   }
 
   // Release the per-function state when we return, whether we succeeded or not.

llvm/test/CodeGen/AArch64/GlobalISel/endian_fallback.ll

@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple aarch64-unknown-linux-musl -O0 < %s | FileCheck %s --check-prefix=CHECK-LE
+; RUN: llc -mtriple aarch64_be-unknown-linux-musl -O0 < %s | FileCheck %s --check-prefix=CHECK-BE
+
+; Make sure we fall-back to SDAG for BE targets.
+
+define <4 x i6> @foo(float %0, <4 x i6> %1) {
+; CHECK-LE-LABEL: foo:
+; CHECK-LE:       // %bb.0:
+; CHECK-LE-NEXT:    fmov d0, d1
+; CHECK-LE-NEXT:    ret
+;
+; CHECK-BE-LABEL: foo:
+; CHECK-BE:       // %bb.0:
+; CHECK-BE-NEXT:    fmov d0, d1
+; CHECK-BE-NEXT:    ret
+  ret <4 x i6> %1
+}