[X86] Fix EXTRACT_VECTOR_ELT with variable index from v32i16 and v64i8 vector.

Its more profitable to go through memory (1 cycles throughput)
than using VMOVD + VPERMV/PSHUFB sequence ( 2/3 cycles throughput) to implement EXTRACT_VECTOR_ELT with variable index.
IACA tool was used to get performace estimation (https://software.intel.com/en-us/articles/intel-architecture-code-analyzer)
For example for var_shuffle_v16i8_v16i8_xxxxxxxxxxxxxxxx_i8 test from vector-shuffle-variable-128.ll I get 26 cycles vs 79 cycles. 
Removing the VINSERT node, we don't need it any more.

Differential Revision: https://reviews.llvm.org/D29690

llvm-svn: 295660