RegisterCoalescer: Fix valuesIdentical() in some subrange merge cases.

I got confused and assumed SrcIdx/DstIdx of the CoalescerPair is a
subregister index in SrcReg/DstReg, but they are actually subregister
indices of the coalesced register that get you back to SrcReg/DstReg
when applied.

Fixed the bug, improved comments and simplified code accordingly.

Testcase by Tom Stellard!

llvm-svn: 225415