[AArch64] Static (de)allocation of SVE stack objects.

Adds support to AArch64FrameLowering to allocate fixed-stack SVE objects.

The focus of this patch is purely to allow the stack frame to
allocate/deallocate space for scalable SVE objects. More dynamic
allocation (at compile-time, i.e. determining placement of SVE objects
on the stack), or resolving frame-index references that include
scalable-sized offsets, are left for subsequent patches.

SVE objects are allocated in the stack frame as a separate region below
the callee-save area, and above the alignment gap. This is done so that
the SVE objects can be accessed directly from the FP at (runtime)
VL-based offsets to benefit from using the VL-scaled addressing modes.

The layout looks as follows:

     +-------------+
     | stack arg   |   
     +-------------+
     | Callee Saves|
     |   X29, X30  |       (if available)
     |-------------| <- FP (if available)
     |     :       |   
     |  SVE area   |   
     |     :       |   
     +-------------+
     |/////////////| alignment gap.
     |     :       |   
     | Stack objs  |
     |     :       |   
     +-------------+ <- SP after call and frame-setup

SVE and non-SVE stack objects are distinguished using different
StackIDs. The offsets for objects with TargetStackID::SVEVector should be
interpreted as purely scalable offsets within their respective SVE region.

Reviewers: thegameg, rovka, t.p.northover, efriedma, rengolin, greened

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D61437

llvm-svn: 373585