[LoopVectorize][NFCI] Use find instead of count

Summary:
Avoid "count" if possible -> use "find" to check for the existence of keys.

Passed llvm test suite.

Reviewers: fhahn, dcaballe, mkuper, rengolin

Reviewed By: fhahn

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51054

llvm-svn: 340563