[AArch64][SVE] Share code across SVE prologue/epilogue implementations (NFCI) (#162253)

This patch refactors the SVE prologue and epilogue to use a common code
path for both the "Default" and "Split" SVE layouts.

This also factors some code into common helpers, such as:
- `partitionSVECS()` (which returns the MI ranges of the PPR/ZPR
   callee-saves)
- `getSVEStackFrameSizes()` (which returns the sizes of the PPR/ZPR
   locals/callee-saves)

Since most split-SVE layout code is now shared with the default path,
there should be much less code that is only tested when split-SVE is
enabled.

Author: MacDue

llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h

@@ -315,6 +315,8 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
   }
 
   void setStackSizeSVE(uint64_t ZPR, uint64_t PPR) {
+    assert(isAligned(Align(16), ZPR) && isAligned(Align(16), PPR) &&
+           "expected SVE stack sizes to be aligned to 16-bytes");
     StackSizeZPR = ZPR;
     StackSizePPR = PPR;
     HasCalculatedStackSizeSVE = true;
@@ -425,6 +427,8 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
 
   // Saves the CalleeSavedStackSize for SVE vectors in 'scalable bytes'
   void setSVECalleeSavedStackSize(unsigned ZPR, unsigned PPR) {
+    assert(isAligned(Align(16), ZPR) && isAligned(Align(16), PPR) &&
+           "expected SVE callee-save sizes to be aligned to 16-bytes");
     ZPRCalleeSavedStackSize = ZPR;
     PPRCalleeSavedStackSize = PPR;
     HasSVECalleeSavedStackSize = true;