Transform slow LEA_B_I_D/LEA_SLOWBASE_I -> LEA_IS_D/LEA_IS iff base == index

The two 3c LEA cases:
    lea D(base, index,1)      -> lea D(,index,2)
    lea D(r13/rbp, index)     -> lea D(,r13/rbp,2) // D maybe zero

Current take 2 instructions to transform. We can do a bit better by
using LEA w.o a base if base == index and scale == 1.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D141980