[llvm][PPC] Add missing case for 'I' asm memory operands

From https://llvm.org/docs/LangRef.html#asm-template-argument-modifiers:

I: Print the letter ‘i’ if the operand is an integer constant,
otherwise nothing. Used to print ‘addi’ vs ‘add’ instructions.

Differential Revision: https://reviews.llvm.org/D103968