Optimization: remove redundant sign-ext instruction on crankshaft

[john-yan]

There are a lot of sign-ext instructions like lgfr/extsw in 64-bit lithium-code-gen for ppc/s390 to ensure that results are 64-bit long. However, This is not necessary. We can delay sign-ext until consuming the resulting 32-bit value. Therefore, if there is a sequence of 32-bit operations, we can reduce the amount of sign-exts and boost performance in general.