Handle instruction lowering without intermediate variables

[maxgillett]

We do not currently support the lowering of an instruction in which the virtual register storing its destination operand is used directly by a subsequent instruction, without first being stored in the stack (see the difference between the two examples below).

%6:anygpr(p0) = G_FRAME_INDEX %stack.2
%7:anygpr(s32) = G_LOAD %2(p0) :: (dereferenceable load (s32) from %ir.3)
%8:anygpr(s32) = G_LOAD %3(p0) :: (dereferenceable load (s32) from %ir.4)
%9:anygpr(s32) = nsw G_MUL %7, %8
G_STORE %9(s32), %6(p0) :: (store (s32) into %ir.5)
%10:anygpr(s32) = G_LOAD %6(p0) :: (dereferenceable load (s32) from %ir.5)
%12:anygpr(s32) = nsw G_ADD %10, %11

%7:anygpr(s32) = G_LOAD %2(p0) :: (dereferenceable load (s32) from %ir.3)
%8:anygpr(s32) = G_LOAD %3(p0) :: (dereferenceable load (s32) from %ir.4)
%9:anygpr(s32) = nsw G_MUL %7, %8
%11:anygpr(s32) = nsw G_ADD %9, %10

As we do not have physical registers, to support this we will need to store the intermediate result (e.g. %9 in the second example) in the stack. We should do this statically, and reuse stack items.