[GlobalISel][TableGen] Take first result for multi-output instructions (#81130)

Previously, tblgen would reject patterns where one of its nested
instructions produced more than one result. These arise when the
instruction definition contains 'outs' as well as 'Defs'. This patch
fixes that by always taking the first result, which is how these
situations are handled in SelectionIDAG.

Original patch: https://reviews.llvm.org/D86617
Continued as: https://github.com/llvm/llvm-project/pull/81130