Thumb2: Clean up 16-bit LDR/STR detection.

The 16-bit LDR/STR sp, #N can be used for N = (imm8 << 2)
but it was previously used only for N < 128. This cleans up
the 16-bit instruction availability calculation for this
case as well as for byte and half word loads and stores.
However, only the LDR/STR sp, #N has impact on the code we
currently generate in Optimizing.

Reduces aosp_hammerhead-userdebug boot.oat by 108KiB.

Test: Run ART test suite on Nexus 5.
Change-Id: Ie06d130a48efad10c23f8ff2596a51efd98d0761