[IfConversion] Fix diamond conversion with unanalyzable branches.

The code was incorrectly counting the number of identical
instructions, and therefore tried to predicate an instruction which
should not have been predicated.  This could have various effects: a
compiler crash, an assembler failure, a miscompile, or just
generating an extra, unnecessary instruction.

Instead of depending on TargetInstrInfo::removeBranch, which only
works on analyzable branches, just remove all branch instructions.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43121 and
https://bugs.llvm.org/show_bug.cgi?id=41121 .

Differential Revision: https://reviews.llvm.org/D67203