[GlobalISel] Do not crash on widening vector result

Function buildCopyToRegs did not handle properly the case when it should
make wider vector result. It happened, for example, in a function that
returns value of type <2 x f32>, which should be widen to <4 x f32> to
fit XMM register. The function eventually calls
MachineIRBuilder.buildUnmerge, which does not expect that only one
destination register is specified.

Now this case is treated specifically in buildCopyToRegs.

Differential Revision: https://reviews.llvm.org/D128546