Make GEPs not suck so much:

* Don't emit the Index * ElementSize multiply if Index is a constant.
* Use a shift, not a multiply, if ElementSize is 1/2/4/8.
* If ElementSize fits in the immediate field of SMUL, then put it there.

Fix a bug where struct offsets might be truncated (ConstantSInt::get is
now used instead of ConstantInt::get).

llvm-svn: 18792