GlobalIsel: Fix fma combine when one of the operands comes from unmerge

Fma combine assumes that MRI.getVRegDef(Reg)->getOperand(0).getReg() = Reg
which is not true when Reg is defined by instruction with multiple defs
e.g. G_UNMERGE_VALUES.
Fix is to keep register and the instruction that defines register in
DefinitionAndSourceRegister and use when needed.

Differential Revision: https://reviews.llvm.org/D117032