ARM assembler: fix undefined behaviour in fits_shifter
Bit-shifts have undefined behaviour if the shift amount is greater
or equal to the width of the type.
In this case the code would do imm32 >> 32 when rot == 0.
A newer version of Clang unrolled the loop, optimized the first
iteration away, causing the test suite to fail with:
# Fatal error in ../src/arm/assembler-arm.cc, line 1212
# Check failed: !rn.is(ip).
as well as crashing when running Chromium tests on Android (at least
we think this was the cause, see the bug).