[IfConversion] Disallow TrueBB == FalseBB for valid diamonds

Summary:
This fixes PR45302.

Previously the case

     BB1
     / \
    |   |
   TBB FBB
    |   |
     \ /
     BB2

was treated as a valid diamond also when TBB and FBB was the same basic
block. This then lead to a failed assertion in IfConvertDiamond.

Since TBB == FBB is quite a degenerated case of a diamond, we now
don't treat it as a valid diamond anymore, and thus we will avoid the
trouble of making IfConvertDiamond handle it correctly.

Reviewers: efriedma, kparzysz

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D77651