[ARM][CGP] Negative constant operand handling

While mutating instructions, we sign extended negative constant
operands for binary operators that can safely overflow. This was to
allow instructions, such as add nuw i8 %a, -2, to still be able to
perform a subtraction. However, the code to handle constants doesn't
take into consideration that instructions, such as sub nuw i8 -2, %a,
require the i8 -2 to be converted into i32 254.

This is a relatively simple fix, but I've taken the time to
reorganise the code a bit - mainly that instructions that can be
promoted are cached and splitting up the Mutate function.

Differential Revision: https://reviews.llvm.org/D53972

llvm-svn: 345840