Several fixes and enhancements to the PPC32 backend.

1. Fix an illegal argument to getClassB when deciding whether or not to
   sign extend a byte load.

2. Initial addition of isLoad and isStore flags to the instruction .td file
   for eventual use in a scheduler.

3. Rewrite of how constants are handled in emitSimpleBinaryOperation so
   that we can emit the PowerPC shifted immediate instructions far more
   often.  This allows us to emit the following code:

int foo(int x) { return x | 0x00F0000; }

_foo:
.LBB_foo_0:     ; entry
        ; IMPLICIT_DEF
        oris r3, r3, 15
        blr

llvm-svn: 16826