[MIRParser] Accept overloaded intrinsic names w/o type suffixes

Function::lookupIntrinsicID is somewhat forgiving as it comes to
overloaded intrinsics' names: it returns an ID as soon as the name
provided has a prefix that matches a registered intrinsic's name w/o
actually checking that the rest of the name encodes all the concrete arg
types, let alone that those types are compatible with the intrinsic's
definition.

That's probably fine and comes in handy in MIR serialization: we don't
care about IR types at MIR level and every intrinsic should be
selectable based on its ID and low-level types (LLTs) of its operands,
including the overloaded ones, so there is no point in serializing
mangled IR types as part of the intrinsic's name.

However, lookupIntrinsicID is somewhat inconsistent in its forgiveness:
if the name provided is actually an exact match, it will refuse to
return the ID if the intrinsic is overloaded. There is probably no
real reason for that and it renders MIRParser incapable to deserialize
MIR MIRPrinter serialized.

This commit fixes it.

Reviewers: rnk, aditya_nandakumar, qcolombet, thegameg, dsanders,
marcello.maggioni

Reviewed By: bogner

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D43267

llvm-svn: 326387