1) In makeHelperCall(), function pointers that are created should have
type IceType_i32, not the functions' own return type.

2) In legalize(), change the name of WillHaveRegister to
MustHaveRegister. Add a comment to clarify the condition being computed.

3) In legalize(), add an assert to make sure that vector "constants"
don't get legalized (other than undef). There should be no constants of
vector type.

4) In copyToReg(), replace an unnecessary use of Src->getType().

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/385133006