LowerTypeTests: Generate simpler IR for br(llvm.type.test, then, else).

This makes it so that the code quality for CFI checks when compiling
with -O2 and linking with --lto-O0 is similar to that of the rest of
the code.

Reduces the size of a chrome binary built with -O2/--lto-O0 by
about 750KB.

Differential Revision: https://reviews.llvm.org/D33925

llvm-svn: 304921