[GlobalISel] Avoid invalidating ValToVReg when translating no-op bitcast.

When we translate a no-op (same type) bitcast, we try to be clever and
only emit a COPY if we already assigned a vreg to the defined value.
However, when we didn't, we tried to assign to a reference into the
ValToVReg DenseMap, even though the RHS of the assignment
(getOrCreateVReg) could potentially grow that DenseMap, invalidating the
reference.

Avoid that by getting the source vreg first.
I audited the rest of the translator; this is the only tricky case.

The test is quite unwieldy, as the problem is caused by the DenseMap
growing, which happens after the 47th mapped value.

llvm-svn: 297208