[mlir][Transforms] Dialect Conversion: Simplify block conversion API (#94866)

This commit simplifies and improves documentation for the part of the
`ConversionPatternRewriter` API that deals with signature conversions.

There are now two public functions for signature conversion:
* `applySignatureConversion` converts a single block signature. This
function used to take a `Region *` (but converted only the entry block).
It now takes a `Block *`.
* `convertRegionTypes` converts all block signatures of a region.

`convertNonEntryRegionTypes` is removed because it is not widely used
and can easily be expressed with a call to `applySignatureConversion`
inside a loop. (See `Detensorize.cpp` for an example.)

Note: For consistency, `convertRegionTypes` could be renamed to
`applySignatureConversion` (overload) in the future. (Or
`applySignatureConversion` renamed to `convertBlockTypes`.)

Also clarify when a type converter and/or signature conversion object is
needed and for what purpose.

Internal code refactoring (NFC) of `ConversionPatternRewriterImpl` (the
part that deals with signature conversions). This part of the codebase
was quite convoluted and unintuitive.

From a functional perspective, this change is NFC. However, the public
API changes, thus not marking as NFC.

Note for LLVM integration: When you see
`applySignatureConversion(region, ...)`, replace with
`applySignatureConversion(region->front(), ...)`. In the unlikely case
that you see `convertNonEntryRegionTypes`, apply the same changes as
this commit did to `Detensorize.cpp`.

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Co-authored-by: Markus Böck <markus.boeck02@gmail.com>

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