[MIRParser][GlobalISel] Parsing vector pointer types (<M x pA>)

MIParser wasn't able to parse LLTs like `<4 x p0>`, fixing that.

Reviewers: qcolombet t.p.northover aditya_nandakumar

Reviewed By: qcolombet

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D46490

llvm-svn: 331712