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

llvm-svn: 22811