[LV] Use SmallVector instead of DenseMap in calculateRegisterUsage (NFC).

We assign indices sequentially for seen instructions, so we can just use
a vector and push back the seen instructions. No need for using a
DenseMap.

Reviewers: hsaito, rengolin, nadav, dcaballe

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D53089

llvm-svn: 344233