[PowerPC] Correctly compute 64-bit offsets in fast isel

PPCSimplifyAddress contains this code:

  IntegerType *OffsetTy = ((VT == MVT::i32) ? Type::getInt32Ty(*Context)
                                            : Type::getInt64Ty(*Context));

to determine the type to be used for an index register, if one needs
to be created.  However, the "VT" here is the type of the data being
loaded or stored, *not* the type of an address.  This means that if
a data element of type i32 is accessed using an index that does not
not fit into 32 bits, a wrong address is computed here.

Note that PPCFastISel is only ever used on 64-bit currently, so the type
of an address is actually *always* MVT::i64.  Other parts of the code,
even in this same PPCSimplifyAddress routine, already rely on that fact.
Thus, this patch changes the code to simply unconditionally use
Type::getInt64Ty(*Context) as OffsetTy.

llvm-svn: 265023