[X86] Introduce WriteFLDC for x87 constant loads.

Summary:
{FLDL2E, FLDL2T, FLDLG2, FLDLN2, FLDPI} were using WriteMicrocoded.

 - I've measured the values for Broadwell, Haswell, SandyBridge, Skylake.
 - For ZnVer1 and Atom, values were transferred form InstRWs.
 - For SLM and BtVer2, I've guessed some values :(

Reviewers: RKSimon, craig.topper, andreadb

Subscribers: gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D47585

llvm-svn: 333656