Remove a very old instcombine where we would turn sequences of selects into
logical operations on the i1's driving them.  This is a bad idea for every
target I can think of (confirmed with micro tests on all of: x86-64, ARM,
AArch64, Mips, and PowerPC) because it forces the i1 to be materialized into
a general purpose register, whereas consuming it directly into a select generally
allows it to exist only transiently in a predicate or flags register.

Chandler ran a set of performance tests with this change, and reported no
measurable change on x86-64.