Use logarithmic units for basic block alignment.

This was actually a bit of a mess. TLI.setPrefLoopAlignment was clearly
documented as taking log2(bytes) units, but the x86 target would still
set a preferred loop alignment of '16'.

CodePlacementOpt passed this number on to the basic block, and
AsmPrinter interpreted it as bytes.

Now both MachineFunction and MachineBasicBlock use logarithmic
alignments.

Obviously, MachineConstantPool still measures alignments in bytes, so we
can emulate the thrill of using as.

llvm-svn: 145889