Split imm16Shifted into a sext/zext form for 64-bit support.
Add some patterns for immediate formation.  For example, we now compile this:

static unsigned long long Y;
void test3() {
  Y = 0xF0F00F00;
}

into:

_test3:
        li r2, 3840
        lis r3, ha16(_Y)
        xoris r2, r2, 61680
        std r2, lo16(_Y)(r3)
        blr

GCC produces:

_test3:
        li r0,0
        lis r2,ha16(_Y)
        ori r0,r0,61680
        sldi r0,r0,16
        ori r0,r0,3840
        std r0,lo16(_Y)(r2)
        blr

llvm-svn: 28883