[SystemZ] Implement ISD::BITCAST for fp128 -> i128.

The type legalizer has by default no method of doing this bitcast other than
storing and reloading the value from stack.

This patch implements a custom lowering of this operation using extractions
of subregs (z13 and earlier using FP128 register pairs), or of vector
elements (with 'vector enhancements 1' using VR128 FP registers).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D110346