[mlir][Vector] Update VectorEmulateNarrowType.cpp

[banach-space]

This PR aims at improving "VectorEmulateNarrowType.cpp". This is mainly
minor refactoring, no major functional changes are made/added.
Implements #123630.

CHANGE 1
Renames srcBits/dstBits to oldBits/newBits to improve consistency in
naming within the file. This is illustrated below:

  // Extracted from VectorEmulateNarrowType.cpp
  Type oldElementType = op.getType().getElementType();
  Type newElementType = convertedType.getElementType();

  // BEFORE (mixing old/new and src/dst):
  // int srcBits = oldElementType.getIntOrFloatBitWidth();
  // int dstBits = newElementType.getIntOrFloatBitWidth();

  // AFTER (consistently using old/new):
  int oldBits = oldElementType.getIntOrFloatBitWidth();
  int newBits = newElementType.getIntOrFloatBitWidth();

Also adds some comments and unifies related "rewriter notification"
messages.

CHANGE 2
Renames the variable "scale". Note, "scale" could mean either:

• "original-elements-per-emulated-type", or
• "emulated-elements-per-original-type".

While from the context it is clear that it's always the former (original
type is always a sub-byte type and the emulated type is usually i8),
this PR reduces the cognitive load by making this clear.

CHANGE 3
Replaces isUnalignedEmulation with isFullyAligned

Note, isUnalignedEmulation is always computed following a
"per-element-alignment" condition:

// Check per-element alignment.
if (newBits % oldBits != 0) {
  return rewriter.notifyMatchFailure(op, "unalagined element types");
}

// (...)

bool isUnalignedEmulation = origElements % elementsPerContainerType != 0;

Given that isUnalignedEmulation captures only one of two conditions
required for "full alignment", it should be re-named as
isPartiallyUnalignedEmulation. Instead, I've flipped the condition and
renamed it as isFullyAligned:

bool isFullyAligned = origElements % elementsPerContainerType == 0;

CHANGE 4
Unifies various comments throughout the file (for consistency).

CHANGE 5
Adds new comments throughout the file and adds TODOs where high-level
comments are missing.

CHANGE 6
Update alignedConversionPrecondition (1):

This method didn't require the vector type for the "destination"
argument. The underlying element type is sufficient. The corresponding
argument has been renamed as multiByteScalarTy - this is meant as the
multi-byte emulated type (i8, i16, i32, etc).

CHANGE 7
Update alignedConversionPrecondition (2):

In #121298, we replaced dstElemBitwidt in this calculation:

  const int numSrcElemsPerDestElem = dstElemBitwidth / srcElemBitwidth;

with the hard-coded value of 8:

  const int numSrcElemsPerDestElem = 8 / srcElemBitwidth;

That was correct as for the patterns for which this hook was/is used:

• RewriteAlignedSubByteIntExt,
• RewriteAlignedSubByteIntTrunc.

The destination type (or, more precisely, the emulated type) was always
i8.

In this PR, I am switching back to a more generic approach - the
calculation should take into account the bit-width of the emulated type.

Note that at the call sites I am passing i8 as the emulated type, so the
end-result is effectively identical. However, the intent is clearer, i.e.,
the underlying value is 8 because the emulated type happens to be i8
(as opposed using a magic number).

CHANGE 8
Update alignedConversionPrecondition (3):

The final check has been replaced with a new helper method,
isSubByteVecFittable. This new method is also re-used within the code
and hopefully will allow us more code re-use moving forward (to avoid
re-implementing the same condition).

[banach-space]

Combines/squashes:

• [mlir][Vector] Update VectorEmulateNarrowType.cpp (1/N) #123526
• [mlir][Vector] Update VectorEmulateNarrowType.cpp (2/N) #123527
• [mlir][Vector] Update VectorEmulateNarrowType.cpp (3/N) #123528
• [mlir][Vector] Refactor VectorEmulateNarrowType.cpp #123529

Waiting for more reviewers' feedback to see whether to abandon ^^^ in favour of this one. I am happy either way.

[banach-space]

There's only PR left in the series, so this is no longer required.