[X86] Remove X86 specific dag nodes for RDTSC/RDTSCP/RDPMC. NFCI

This patch removes the following dag node opcodes from namespace X86ISD:

RDTSC_DAG,
RDTSCP_DAG,
RDPMC_DAG

The logic that expands RDTSC/RDPMC/XGETBV intrinsics is basically the same. The
only differences are:

    RDTSC/RDTSCP don't implicitly read ECX.
    RDTSCP also implicitly writes ECX.

I moved the common expansion logic into a helper function with the goal to get
rid of code repetition. That helper is now used for the expansion of
RDTSC/RDTSCP/RDPMC/XGETBV intrinsics.

No functional change intended.

Differential Revision: https://reviews.llvm.org/D59547

llvm-svn: 356546