Implement a couple improvements:

Implement a couple improvements:
Remove dead code in ISD::Constant handling
Add support for add long, imm16

We now codegen 'long long foo(long long a) { return ++a; }'
as:
addic r4, r4, 1
addze r3, r3
blr

instead of:
li r2, 1
li r5, 0
addc r2, r4, r2
adde r3, r3, r5
blr

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22811 91177308-0d34-0410-b5e6-96231b3b80d8