aarch64: Fix/avoid undefinedness in aarch64_classify_index [PR100212]

aarch64: Fix/avoid undefinedness in aarch64_classify_index [PR100212]

The problem here is we don't check the return value of exact_log2
and always use that result as shifter. This fixes the issue by avoiding
the shift if the value was `-1` (which means the value was not exact a power of 2);
in this case we could either check if the values was equal to -1 or not equal to because
we then assign -1 to shift if the constant value was not equal. I chose `!=` as
it seemed to be more obvious of what the code is doing.

Committed as obvious after a build/test for aarch64-linux-gnu.

gcc/ChangeLog:

	PR target/100212
	* config/aarch64/aarch64.cc (aarch64_classify_index): Avoid
	undefined shift after the call to exact_log2.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>