[UBSan][MIPS] Fix cast-overflow tests for mips big endian

This fixes the bug https://llvm.org/bugs/show_bug.cgi?id=24152
The float value resides in the first 4 bytes of ValueHandle for both mips and mipsel.

Reviewers: dsanders, samsonov
Subscibers: rsmith, hans, mohit.bhakkad, jaydeep, llvm-commits
Differential: http://reviews.llvm.org/D11448
llvm-svn: 243384