i965/vec4: Add a helper function to emit VS_OPCODE_PULL_CONSTANT_LOAD

i965/vec4: Add a helper function to emit VS_OPCODE_PULL_CONSTANT_LOAD

There were three places in the visitor that had a similar chunk of
code to emit the VS_OPCODE_PULL_CONSTANT_LOAD opcode using a register
for the offset. This patch combines the chunks into a helper function
to reduce the code duplication. It will also be useful in the next
patch to expand what happens on Gen9+. This shouldn't introduce any
functional changes.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>