nir/algebraic: Add support for unsized conversion opcodes

nir/algebraic: Add support for unsized conversion opcodes

All conversion opcodes require a destination size but this makes
constructing certain algebraic expressions rather cumbersome.  This
commit adds support to nir_search and nir_algebraic for writing
conversion opcodes without a size.  These meta-opcodes match any
conversion of that type regardless of destination size and the size gets
inferred from the sizes of the things being matched or from other
opcodes in the expression.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>