[DAG] BUILD_VECTOR: absorb ZERO_EXTEND of a single first operand if all other ops...

[DAG] BUILD_VECTOR: absorb ZERO_EXTEND of a single first operand if all other ops are zeros

This kind of pattern seems to come up as regressions
with better ZERO_EXTEND_VECTOR_INREG recognition.

For initial implementation, this is quite restricted
to the minimal viable transform, otherwise there are
too many regressions to be dealt with.