[SCEV] Adding a check on outgoing branches of a terminator instr for SCEVBackedgeCond...

[SCEV] Adding a check on outgoing branches of a terminator instr for SCEVBackedgeConditionFolder, NFC.

Summary:
For a given loop, getLoopLatch returns a non-null value
when a loop has only one latch block. In the modified
context adding an assertion to check that both the outgoing branches of
a terminator instruction (of latch) does not target same header.
+
few minor code reorganization.

Reviewers: jbhateja

Reviewed By: jbhateja

Subscribers: sanjoy

Differential Revision: https://reviews.llvm.org/D40460

llvm-svn: 318997