Preserve value numbers when discarding a cast.

Preserve value numbers when discarding a cast.

This is a fix for an assert during CSE. After processing a CSE definition
the tree got morphed. During morphing a cast was discarded by narrowing
the operand of the cast. The value number of the operand was cleared. Later,
when processing a use of the CSE, the code was attempting to process the side effects
in the tree of the use and was encountering a node with no value number resulting
in an assert. The fix is to transfer the value number from the cast node to the narrowed
operand.