[InstCombine][X86] Improved folding of calls to Intrinsic::x86_sse4a_insertqi.

This patch teaches the instruction combiner how to fold a call to 'insertqi' if
the 'length field' (3rd operand) is set to zero, and if the sum between
field 'length' and 'bit index' (4th operand) is bigger than 64.

From the AMD64 Architecture Programmer's Manual:
1. If the sum of the bit index + length field is greater than 64, then the
   results are undefined;
2. A value of zero in the field length is defined as a length of 64.

This patch improves the existing combining logic for intrinsic 'insertqi'
adding extra checks to address both point 1. and point 2.

Differential Revision: http://reviews.llvm.org/D6583