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 on X86. 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.