[GlobalISel] NFCI, Getting GlobalISel ~5% faster

by replacing DenseMap with IndexedMap for LLTs within MRI, as
benchmarked by cross-compiling sqlite3 amalgamation for AArch64
on x86 machine.

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D46809

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333125 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed