[RISCV] Bugfix for floats passed on the stack with the ILP32 ABI on RV32F

f32 values passed on the stack would previously cause an assertion in 
unpackFromMemLoc.. This would only trigger in the presence of the F extension 
making f32 a legal type. Otherwise the f32 would be legalized.

This patch fixes that by keeping LocVT=f32 when a float is passed on the 
stack. It also adds test coverage for this case, and tests that also 
demonstrate lw/sw/flw/fsw will be selected when most profitable. i.e. there is 
no unnecessary i32<->f32 conversion in registers.

llvm-svn: 343756