Fixes some issues related to preservation of regs in the common.decode test, seen on the x86-32 bit suite on latest Ubuntu versions.
Preserves stack reg in a separate data var on the test_modrm16 test.
Preserves callee saved regs in the test_avx512_vex test.
Removes common.decode from ignore list. Verified locally that there was no failure in 1000 runs.
Issue: #4953 (closed)