[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

llvm-svn: 333125