[AArch64][SVE] Spilling/filling of SVE callee-saves.
Implement the spills/fills of callee-saved SVE registers using STR and LDR
instructions.
Also adds the `aarch64_sve_vector_pcs` attribute to specify the
callee-saved registers to be used for functions that return SVE vectors or
take SVE vectors as arguments. The callee-saved registers are vector
registers z8-z23 and predicate registers p4-p15.
The overal frame-layout with SVE will be as follows:
+-------------+
| stack args |
+-------------+
| Callee Saves|
| X29, X30 |
|-------------| <- FP
| SVE Callee | < //////////////
| saved regs | < //////////////
| z23 | < //////////////
| : | < // SCALABLE //
| z8 | < //////////////
| p15 | < /// STACK ////
| : | < //////////////
| p4 | < //// AREA ////
+-------------+ < //////////////
| : | < //////////////
| SVE locals | < //////////////
| : | < //////////////
+-------------+
|/////////////| alignment gap.
| : |
| Stack objs |
| : |
+-------------+ <- SP after call and frame-setup
Reviewers: cameron.mcinally, efriedma, greened, thegameg, ostannard, rengolin
Reviewed By: ostannard
Differential Revision: https://reviews.llvm.org/D68996