[PowerPC][Future] Remove redundant r2 save and restore for indirect call

Currently an indirect call produces the following sequence on PCRelative mode:

extern void function( );
extern void (*ptrfunc) ( );

void g() {
    ptrfunc=function;
}

void f() {
    (*ptrfunc) ( );
}

Producing

paddi 3, 0, .LC0@PCREL, 1
ld 3, 0(3)
std 2, 24(1)
ld 12, 0(3)
mtctr 12
bctrl
ld 2, 24(1)

Though the caller does not use or preserve r2, it is still saved and restored
across a function call. This patch is added to remove these redundant save and
restores for indirect calls.

Differential Revision: https://reviews.llvm.org/D77749