[GISel] Add support for scalable vectors in getGCDType (#80307)

This function can be called from buildCopyToRegs where at least one of
the types is a scalable vector type. This function crashed because it
did not know how to handle scalable vector types.

This patch extends the functionality of getGCDType to handle when at
least one of the types is a scalable vector. getGCDType between a fixed
and scalable vector is not implemented since the docstring of the
function explains that getGCDType is used to build MERGE/UNMERGE
instructions and we will never build a MERGE/UNMERGE between fixed and
scalable vectors.

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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>