fix size of loaded bitfields

fix size of loaded bitfields

Loading a bitfield correctly take in account the offset
of the bitfield inside the whole container integer.
But truncating it to the width of the bitfield is not done
or is done very implicitely (because the correct size is not lost).
For example, with the following code:
	struct bfu {
		unsigned int a:3;
	};
	unsigned int get__bfu_a(struct bfu bf) { return bf.a; }

test-linearize gives as output something like:
	get__bfu_a:
		cast.32     %r2 <- (3) %arg1
		ret.32      %r2

We can notice the (3) in the cast instruction but this is misleading
as %arg1 is not 3bit wide.

Fix this by adding the missing truncating cast.
This will then gives something like:
	get__bfu_a:
		cast.3      %r2 <- (32) %arg1
		cast.32     %r3 <- (3) %r2
		ret.32      %r3

Note the truncation could also be done by a and-mask but the cast
is more logical since we're here only changing size and not doing
some arithmetic operations.

Fixes: 1688f039c ("Re-do memory access linearization.")
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>