For some arithmetic assembler methods, instead of checking
IceType_i8 || IceType_i1, only allow IceType_i8 and assert if
an i1 leaked to that stage (should have been vetted earlier
by the bitcode reader / ABI checks). Could have looked up the
type width and isIntegerArithmeticType, etc. in the property table,
but that seemed a bit heavy for just checking one type
(or one of two types).

Also changed some f32 || f64 checks into just using
isScalarFloatingType() which looks things up in a property table.
Could alternatively just keep it as an simple f32 || f64 check,
and I could change isScalarFloatingType()'s implementation.

In some places where we assume something is either i32 or i64
and do a select, change that into using a helper function
so that we can do one compare, and then assert. Some of the
asserts are really redundant (already within a branch which
already checked that), but hopefully that disappears if
we compile in release mode.
Similar for f32 or f64 (which happened a lot in the assembler).

BUG=none
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/613483002