[llvm] [RFC][MC] Cache MCRegAliasIterator (PR #93510)

[Pierre van Houtryve via llvm-commits]

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@@ -187,6 +187,9 @@ class MCRegisterInfo {
   DenseMap<MCRegister, int> L2SEHRegs;        // LLVM to SEH regs mapping
   DenseMap<MCRegister, int> L2CVRegs;         // LLVM to CV regs mapping
 
+  mutable std::vector<std::vector<MCPhysReg>> RegAliasesCache;
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Pierre-vh wrote:

@nikic SmallVector is 32 bytes, it has one more pointer for some reason.
Though it means I can use `SmallVector<MCPhysReg, 4>` and store small alias sets inline, so I think it's worth the size increase. 

https://github.com/llvm/llvm-project/pull/93510