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
