[MLIR] Infer locations for block argument conversion

To enable signature conversions to be used in CIRCT, locations should no longer be dropped from block arguments.

Reviewed By: Mogball, springerm

Differential Revision: https://reviews.llvm.org/D157882

Author: nandor

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -724,9 +724,18 @@ Block *ArgConverter::applySignatureConversion(
   Block *newBlock = block->splitBlock(block->begin());
   block->replaceAllUsesWith(newBlock);
 
-  // FIXME: We should map the new arguments to proper locations.
+  // Map all new arguments to the location of the argument they originate from.
   SmallVector<Location> newLocs(convertedTypes.size(),
                                 rewriter.getUnknownLoc());
+  for (unsigned i = 0; i < origArgCount; ++i) {
+    auto inputMap = signatureConversion.getInputMapping(i);
+    if (!inputMap || inputMap->replacementValue)
+      continue;
+    Location origLoc = block->getArgument(i).getLoc();
+    for (unsigned j = 0; j < inputMap->size; ++j)
+      newLocs[inputMap->inputNo + j] = origLoc;
+  }
+
   SmallVector<Value, 4> newArgRange(
       newBlock->addArguments(convertedTypes, newLocs));
   ArrayRef<Value> newArgs(newArgRange);