[SystemZ][FPEnv] Enable strict vector FP extends/truncations

The back-end currently has special DAGCombine code to detect
cases where two floating-point extend or truncate operations
can be combined into a single vector operation.

This patch extends that support to also handle strict FP operations.

Note that currently only the case where both operations have the
same input chain are supported.  This already suffices to cover
the common case where the operations result from scalarizing a
non-legal vector type.  More general cases can be supported in
the future.