[mlir] Add InsertionGuards to OneToNPatternRewriter.

[ingomueller-net]

Summary

This fixes bad behavior of that class that surfaced in
https://reviews.llvm.org/D154299, where calling applySignatureConversion
left the insertion point different from before the call, which broke a
subsequent call to replaceOp. This patch introduces a fix in both
functions, each of which is enough to fix the specific problem in the
aforementioned diff: (1) applySignatureConversion now resets the
insertion point with a guard for the whole function and (2) replace sets
the insertion point to the op that should be replaced (and resets it
with a guard).

mlir/lib/Transforms/Utils/OneToNTypeConversion.cpp

 }
 
 void OneToNPatternRewriter::replaceOp(Operation *op, ValueRange newValues,
                                       const OneToNTypeMapping &resultMapping) {
   // Create a cast back to the original types and replace the results of the
   // original op with those.
   assert(newValues.size() == resultMapping.getConvertedTypes().size());
   assert(op->getResultTypes() == resultMapping.getOriginalTypes());
+  PatternRewriter::InsertionGuard g(*this);
+  setInsertionPointAfter(op);
   SmallVector<Value> castResults =
       buildUnrealizedBackwardsCasts(newValues, resultMapping, *this);
   replaceOp(op, castResults);
 }
 
 Block *OneToNPatternRewriter::applySignatureConversion(
     Block *block, OneToNTypeMapping &argumentConversion) {
+  PatternRewriter::InsertionGuard g(*this);
+
   // Split the block at the beginning to get a new block to use for the
   // updated signature.
   SmallVector<Location> locs;
   argumentConversion.convertLocations(block->getArguments(), locs);
   Block *newBlock =
       createBlock(block, argumentConversion.getConvertedTypes(), locs);
   replaceAllUsesWith(block, newBlock);