Lower casting operations that involve vector types.

Lower casting operations that involve vector types.

Impacted instructions:

bitcast {v4f32, v4i32, v8i16, v16i8} <-> {v4f32, v4i32, v8i16, v16i8}
bitcast v8i1 <-> i8
bitcast v16i1 <-> i16

(There was already code present to handle trivial bitcasts like v16i1 <-> v16i1.)

[sz]ext v4i1 -> v4i32
[sz]ext v8i1 -> v8i16
[sz]ext v16i1 -> v16i8

trunc v4i32 -> v4i1
trunc v8i16 -> v8i1
trunc v16i8 -> v16i1

[su]itofp v4i32 -> v4f32
fpto[su]i v4f32 -> v4i32

Where there is a relatively simple lowering to x86 instructions, it has been used. Otherwise a helper call is used.

Some lowerings require a materialization of a integer vector with 1s in each entry. Since there is no support for vector constant pools, the constant is materialized purely through register operations.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/383303003