[mlir][Vector] Add a rewrite pattern for better low-precision ext(bitcast) expansion

This revision adds a rewrite for sequences of vector `ext(bitcast)` to use a more efficient sequence of
vector operations comprising `shuffle` and `bitwise` ops.

Such patterns appear naturally when writing quantization / dequantization functionality with the vector dialect.

The implementation is 90% a refactoring of the existing `trunci(bitcast)` pattern into a common BitCastRewriter.

The rewrite performs a simple enumeration of each of the bits in the result vector and determines its provenance
in the source vector. The enumeration is used to generate the proper sequence of `shuffle`, `andi`, `ori`
with shifts`.

The rewrite currently only applies to 1-D non-scalable vectors and bails out if the final vector element type is
not a multiple of 8. This is a failsafe heuristic determined empirically: if the resulting type
is not an even number of bytes, further complexities arise that are not improved by this pattern:
the heavy lifting still needs to be done by LLVM.

Author: nicolasvasilache

mlir/include/mlir/Dialect/Vector/Transforms/VectorRewritePatterns.h

@@ -23,6 +23,7 @@ namespace mlir {
 class RewritePatternSet;
 
 namespace arith {
+class AndIOp;
 class NarrowTypeEmulationConverter;
 class TruncIOp;
 } // namespace arith
@@ -309,6 +310,12 @@ FailureOr<Value> rewriteBitCastOfTruncI(RewriterBase &rewriter,
                                         arith::TruncIOp truncOp,
                                         vector::BroadcastOp maybeBroadcastOp);
 
+/// Rewrite a vector `ext(bitcast)` to use a more efficient sequence of
+/// vector operations comprising `shuffle` and `bitwise` ops.
+FailureOr<Value> rewriteExtOfBitCast(RewriterBase &rewriter, Operation *extOp,
+                                     vector::BitCastOp bitCastOp,
+                                     vector::BroadcastOp maybeBroadcastOp);
+
 /// Appends patterns for rewriting vector operations over narrow types with
 /// ops over wider types.
 void populateVectorNarrowTypeRewritePatterns(RewritePatternSet &patterns,