i965/fs: Add a concept of a width to fs_reg

i965/fs: Add a concept of a width to fs_reg

Every register in i965 assembly implicitly has a concept of a "width".
Usually, this is derived from the execution size of the instruction.
However, when writing a compiler it turns out that it is frequently a
useful to have the width explicitly in the register and derive the
execution size of the instruction from the widths of the registers used in
it.

This commit adds a width field to fs_reg along with an effective_width()
helper function.  The effective_width() function tells you how wide the
register effectively is when used in an instruction.  For example, uniform
values have width 1 since the data is not actually repeated, but when used
in an instruction they take on the width of the instruction.  However, for
some instructions (LOAD_PAYLOAD being the notable exception), the width is
not the same.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>