Improve type conversion error propagation/failure during LLVM lowering

Improve type conversion error propagation/failure during LLVM lowering.

BEFORE

```
llvm-mlir/mlir/lib/Conversion/LLVMCommon/TypeConverter.cpp:304: SmallVector<mlir::Type, 5> mlir::LLVMTypeConverter::getMemRefDescriptorFields(mlir::MemRefType, bool): Assertion `isStrided(type) && "Non-strided layout maps must have been normalized away"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
...
```

AFTER
```
<unknown>:0: error: integer overflow during size computation
<unknown>:0: error: Conversion to strided form failed either due to non-strided layout maps (which should have been normalized away) or other reasons
<unknown>:0: error: failed to legalize operation 'gpu.func' that was explicitly marked illegal
<unknown>:0: note: see current operation:
"gpu.func"() ( {
...
```

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D139072

Author: bondhugula

mlir/lib/Conversion/GPUCommon/GPUOpsLowering.cpp

@@ -43,9 +43,11 @@ GPUFuncOpLowering::matchAndRewrite(gpu::GPUFuncOp gpuFuncOp, OpAdaptor adaptor,
   }
 
   // Rewrite the original GPU function to an LLVM function.
-  auto funcType = typeConverter->convertType(gpuFuncOp.getFunctionType())
-                      .template cast<LLVM::LLVMPointerType>()
-                      .getElementType();
+  auto convertedType = typeConverter->convertType(gpuFuncOp.getFunctionType());
+  if (!convertedType)
+    return failure();
+  auto funcType =
+      convertedType.template cast<LLVM::LLVMPointerType>().getElementType();
 
   // Remap proper input types.
   TypeConverter::SignatureConversion signatureConversion(

mlir/lib/Conversion/LLVMCommon/TypeConverter.cpp

@@ -199,6 +199,8 @@ Type LLVMTypeConverter::convertFunctionType(FunctionType type) {
   SignatureConversion conversion(type.getNumInputs());
   Type converted =
       convertFunctionSignature(type, /*isVariadic=*/false, conversion);
+  if (!converted)
+    return {};
   return LLVM::LLVMPointerType::get(converted);
 }
 
@@ -298,8 +300,13 @@ LLVMTypeConverter::convertFunctionTypeCWrapper(FunctionType type) {
 SmallVector<Type, 5>
 LLVMTypeConverter::getMemRefDescriptorFields(MemRefType type,
                                              bool unpackAggregates) {
-  assert(isStrided(type) &&
-         "Non-strided layout maps must have been normalized away");
+  if (!isStrided(type)) {
+    emitError(
+        UnknownLoc::get(type.getContext()),
+        "conversion to strided form failed either due to non-strided layout "
+        "maps (which should have been normalized away) or other reasons");
+    return {};
+  }
 
   Type elementType = convertType(type.getElementType());
   if (!elementType)