[X86] Add a helper lambda to getIntelProcessorTypeAndSubtype to select feature bits...

[X86] Add a helper lambda to getIntelProcessorTypeAndSubtype to select feature bits from the correct 32-bit feature variable.

We have three 32 bit variables containing feature bits. But our
enum is a flat 96 bit space. So we need to pick which of the
variables to use based on the bit value. We used to do this
manually by mentioning the correct variable and subtracting an
offset from the enum. But this is error prone.