Several fixes and enhancements to the PPC32 backend.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16826 91177308-0d34-0410-b5e6-96231b3b80d8