Reduce x86 register context boilerplate.

Summary:
The x86 FPR struct was defined as a struct containing a union between
two members: XSAVE and FXSAVE. This patch makes FPR a union directly to
remove one layer of indirection when trying to access the members.

The initial layout of these two structs is identical, which is
recognised by the fact that XSAVE has FXSAVE as its first member, so we
also considered removing one more layer and leave FPR identical to XSAVE
struct, but stopped short of doing that, as the FPR may be used to store
different layouts in the future (e.g., ones generated by the FSAVE
instruction).

Reviewers: clayborg, krytarowski

Subscribers: emaste, lldb-commits

Differential Revision: https://reviews.llvm.org/D41245

llvm-svn: 320966