Implement insertelement IR instruction
We first lower this, generating a mask and using vselect.  This form is then matched and turned into a predicated store.
This:

  %result = insertelement <16 x i32> %orig, i32 %value, i32 %lane

Turns into this:

	s0 = 1
    s0 = s0 << %lane
    %orig{s0} = %value