[MachineCopyPropagation] Make a SmallVector larger (NFC) (#79106)

This patch makes a SmallVector slightly larger.  We encounter quite a
few instructions with 3 or 4 defs but very few beyond that on X86.

This saves 0.39% of heap allocations during the compilation of a large
preprocessed file, namely X86ISelLowering.cpp, for the X86 target.

Author: kazutakahirata

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -853,7 +853,7 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
     forwardUses(MI);
 
     // Not a copy.
-    SmallVector<Register, 2> Defs;
+    SmallVector<Register, 4> Defs;
     const MachineOperand *RegMask = nullptr;
     for (const MachineOperand &MO : MI.operands()) {
       if (MO.isRegMask())