[EarlyIfConversion] Don't if-convert unconditional branches.

A block ending in an unconditional branch can have two successors if one
is a landing pad.  In practice, I think this only has an effect on
Windows because landing pads are never empty for Itanium unwinding.

(Alternatively, I could add a check to
AArch64InstrInfo::canInsertSelect, but this seems more obvious.)

Differential Revision: https://reviews.llvm.org/D56468

llvm-svn: 351142