[X86] Improve handling of UDIVREM8_ZEXT_HREG/SDIVREM8_SEXT_HREG to support 64-bit extensions.

If the extend type is 64-bits, emit a 32-bit -> 64-bit extend after the UDIVREM8_ZEXT_HREG/UDIVREM8_SEXT_HREG operation.

This gives a shorter encoding for the second extend in the sext case, and allows us to completely remove the second extend in the zext case.

This also adds known bit and num sign bits support for UDIVREM8_ZEXT_HREG/SDIVREM8_SEXT_HREG.

Differential Revision: https://reviews.llvm.org/D38275

llvm-svn: 316702