Remove a unnecessary hasLoopInvariantOperands check in loop sink.

Summary:
Preheader instruction's operands will always be invariant w.r.t. the loop which its the preheader
for.

Memory aliases are handled in canSinkOrHoistInst.

Reviewers: danielcdh, davidxl

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D28270

llvm-svn: 291132