rs6000-tdep.c:skip_prologue avoid negative left shift

rs6000-tdep.c:skip_prologue avoid negative left shift

the rs6000-tdep.c::skip_prologue function has the following code:

          unsigned int all_mask = ~((1U << fdata->saved_gpr) - 1);

          /* Not a recognized prologue instruction.
             Handle optimizer code motions into the prologue by continuing
             the search if we have no valid frame yet or if the return
             address is not yet saved in the frame.  Also skip instructions
             if some of the GPRs expected to be saved are not yet saved.  */
          if (fdata->frameless == 0 && fdata->nosavedpc == 0
              && (fdata->gpr_mask & all_mask) == all_mask)
            break;

The problem is that fdata->saved_gpr is initialized to -1, and so,
if no instruction is found in the function's prologue that causes us
to set that field to a non-negative value, the sanitizer crashes
with the following message:

    rs6000-tdep.c:1965:34: runtime error: shift exponent -1 is negative

This patch fixes the issue the by only doing the shift if saved_gpr
is not negative. When saved_gpr is negative, we actually don't need
the shift.

gdb/ChangeLog:

        * rs6000-tdep.c (skip_prologue): Fix potential negative left
        shifting.

Tested on ppc-linux native.
Also tested on ppc-elf (baremetal) using AdaCore's testsuite.