[RegisterBankInfo] Remove overly-agressive asserts

Summary:
We were asserting in RegisterBankInfo if RBI.copyCost() returns
UINT_MAX.  This is OK for RegBankSelect::Mode::Fast since we only
try one instruction mapping and can't recover from this, but for
RegBankSelect::Mode::Greedy we will be considering multiple
instruction mappings, so we can recover if we see a UNIT_MAX copy
cost.

The copy cost for one pair of register banks in the AMDGPU backend
will be UNIT_MAX, so this patch will prevent AMDGPU tests from
breaking.

Reviewers: ab, qcolombet, t.p.northover, dsanders

Reviewed By: qcolombet

Subscribers: tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D33144

llvm-svn: 303043