Set the floating point status register as reserved

Summary:
This patch sets the FPSW (X87 floating-point status register) as a reserved
physical register and fix the test failure caused by [[ https://reviews.llvm.org/D68854| D68854 ]].

Before this patch, some tests will fail because it implicit uses FPSW without
define it. Setting the FPSW as a reserved physical register will skip liveness
analysis because it is always live.

Reviewers: pengfei, craig.topper

Reviewed By: craig.topper

Subscribers: craig.topper, hiraditya, llvm-commits

Patch by LiuChen.

Differential Revision: https://reviews.llvm.org/D69784