[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.