Handle undefined L-value selects

L-value bit selects were incorrectly converting an undefined index to 0.

L-value part selects were asserting if an undefined index was given.

This patch unifies how all these are handle (including indexed part selects).

The base is set to an undefined value and an appropriate width is used.
A warning message is always printed since this is not a simple out of
range issue. It is the responsibility of the code generator to skip the
assignment, but we always want to execute the R-value since it could have
a side effect.