[PowerPC] Mark all instructions as non-cheap for MachineLICM

[PowerPC] Mark all instructions as non-cheap for MachineLICM

MachineLICM uses a callback named hasLowDefLatency to determine if an
instruction def operand has a 'low' latency. If all relevant operands have a
'low' latency, the instruction is considered too cheap to hoist out of loops
even in low-register-pressure situations. On PowerPC cores, both the embedded
cores and the others, there is no reason to believe that this is a good choice:
all instructions have a cost inside a loop, and hoisting them when not limited
by register pressure is a reasonable default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225471 91177308-0d34-0410-b5e6-96231b3b80d8