This commit adds 1:N support to
`ConversionPatternRewriter::replaceUsesOfBlockArgument`. This was one of
the few remaining dialect conversion APIs that does not support 1:N
conversions yet.
This commit also reuses `replaceUsesOfBlockArgument` in the
implementation of `applySignatureConversion`. This is in preparation of
the One-Shot Dialect Conversion refactoring. The goal is to bring the
`applySignatureConversion` implementation into a state where it works
both with and without rollbacks. To that end, `applySignatureConversion`
should not directly access the `mapping`.
b1b8f67eab