This commit adds 1:N support to `SignatureConversion::remapInputs`. This
API allows users to replace a block argument with multiple replacement
values. (And the block argument is dropped.) The API already supported
"bbarg --> multiple bbargs" mappings, but "bbarg --> multiple SSA
values" was missing.
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Co-authored-by: Markus Böck <markus.boeck02@gmail.com>