Fix indentation using clang-format for Spiller.h. NFC

Spiller.h had indentation that doesn't match LLVM guidelines. Fixing it

Reviewers: qcolombet, arsenm

Subscribers: MatzeB, wdng, hiraditya, llvm-commits, qcolombet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75830