Adjust MaxAtomicInlineWidth for i386/i486 targets.

This is to fix the bug reported in https://bugs.llvm.org/show_bug.cgi?id=34347#c6.
Currently, all  MaxAtomicInlineWidth of x86-32 targets are set to 64. However,
i386 doesn't support any cmpxchg related instructions. i486 only supports cmpxchg.
So in this patch MaxAtomicInlineWidth is reset as follows:
For i386, the MaxAtomicInlineWidth should be 0 because no cmpxchg is supported.
For i486, the MaxAtomicInlineWidth should be 32 because it supports cmpxchg.
For others 32 bits x86 cpu, the MaxAtomicInlineWidth should be 64 because of cmpxchg8b.

Differential Revision: https://reviews.llvm.org/D42154

llvm-svn: 323281