Fix a compiler bug caused by Utils::IsPowerOfTwo treating zero as a power of two.

Fix a compiler bug caused by Utils::IsPowerOfTwo treating zero as a power of two.

The IsPowerOfTwo function is used together with ShiftForPowerOfTwo.  Both function
do not work with zero. This caused the optimizing compiler to generate invalid code
for the expression

x ? 0 : 0

where it assumed that if one of the constants is a power-of-two, it can
be computed by (1 << n). We check for 0 in a number of places, but instead
I decided to fix Utils::IsPowerOfTwo itself and remove unnecessary checks
for the zero case.

TEST=tests/language/vm/if_conversion_vm_test.dart, runtime/vm/utils_test.cc
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27033

[Kevin Millikin (Google), 2013-09-03 08:11:42]

LGTM.

[Florian Schneider, 2013-09-03 09:29:04]

Committed patchset #2 manually as r27033 (presubmit successful).