Fix ImmLogic.invert(), and with it, fcopysign and float_misc test.

Previously, `fcopysign` was mysteriously failing to pass the
`float_misc` spec test. This was tracked down to bad logical-immediate
masks used to separate the sign and not-sign bits. In particular, the
masks for the and-not operations were wrong. The `invert()` function on
an `ImmLogic` immediate, it turns out, assumed every immediate would be
used by a 64-bit instruction; `ImmLogic` immediates are subtly different
for 32-bit instructions. This change tracks the instruction size (32 or
64 bits) intended for use with each such immediate, and passes it back
into `maybe_from_u64` when computing the inverted immediate.

Addresses several of the failures (`float_misc`, `f32_bitwise`) for
 #1521 (test failures) and presumably helps #1519 (SpiderMonkey
integration).

build.rs

@@ -243,8 +243,6 @@ fn should_panic(testsuite: &str, testname: &str) -> bool {
         | ("multi_value", "call")
         | ("spec_testsuite", "call")
         | ("spec_testsuite", "conversions")
-        | ("spec_testsuite", "f32_bitwise")
-        | ("spec_testsuite", "float_misc")
         | ("spec_testsuite", "i32")
         | ("spec_testsuite", "i64")
         | ("spec_testsuite", "int_exprs")